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I have a simple setup with a decent amp, mixer, wireless mics, and two nice speakers.  I have an additional amp and a subwoofer.  Here's my question:

Can I bridge the amps and feed the subwoofer off the other amp and get the boom, or do I need to use my dusty crossover (which I've never been able to figure out)???

Please and thank you.

Completely depends on the amp model if it has a built in crossover or if the sub has a built in crossover.  If neither, then you would need an external crossover which isn't real confusing.
You would typically bridge 1 amp for the sub & continue to hook up the other amp & speakers to the tops.  Again, this is completely dependant on the model amp & model sub.  Some amps can't be bridged, some can ONLY be bridged to 8 ohms, some subs are 4 ohms.  More info.

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Both amps are Behringers.  EP1500 (which I was thinking of using to power the sub) and EP2500 which powers my mains.  They can be bridged (I believe at 8 ohms).

What's the typical setup for this with the crossover?  I've never used the crossover before.  The manual talks about mono bridging and bi-amping.  ?????

What model is the sub?  Is it 4 ohm or 8 ohm.  Watt is the power rating continuous/program/peak.  

Keep using the EP2500 for the mains.  Bridge the 1500 for the sub - you will need to flip some of the little white switches on the back of the amp (check your manual or go to Behringer for one, page 8 shows the switch positions for bridge mode operation - or ask here), using input 1 only.  As far as crossover hookup, again, need the model, is it a 2 way or 3 way?  Some connections & adjustments are different than others - some have different output configurations too.

Typical setup for a 2 way with a sub summed output would be running from mixer outs (through any eq processing if any) to the crossover input.  The highs would feed to your EP2500 & the summed sub out would feed the bridged 1500.  Set the frequency at around 75-100 hz.  This will send everything lower than 75-100 hz to the sub & everything higher to the mains.

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My speakers are three way Cerwin Vegas; the crossover may be two way.  I don't have it in front of me to verify.  

My sub is a JRX118S 18" Compact Subwoofer:

Specifications  
Frequency Range (-10 dB):  38 Hz - 300 Hz    
Frequency Response (±3 dB):  55 Hz - 300 Hz  
Sensitivity :   101 dB SPL  
Nominal Impedence:  4 ohms  
*Power Capacity:  350 watts  
*Peak Power Capacity:  1400 watts  
Recommended Amplifier Power:  350 watts to 700 watts into 4 ohms  
Recommended Crossover Settings:  30 Hz HPF, 24 dB / octave; 80 - 100 Hz LPF, 24 dB / octave  
Maximum SPL:   127 dB    
Dimensions (H x W x D):   597 mm x 516 mm x 559 mm
(23.5 in x 20.3 in x 22 in)  
Weight:  32.2 kg (71 lbs)  
Low Frequency Driver:  JBL 2043-G  
Input Connectors:   Neutrik® Speakon® NL-4 (x2); ¼" TS phone jack (x1); parallel

Ok, the extra amp you have would be a little too much for this sub in bridged mode.  The sub is a 4 ohm speaker, you wouldn't want much more than 700 watts at 4 ohms.  The amp pushes 1400 watts at 4 ohms in bridge mode.
You would almost be better off switching the mains to the 1500 & just use 1 channel of the 2500 for the sub - not bridged.  The 2500 pushes 750 watts per channel at 4 ohms.  
The Cerwins you have need 350 watts at 8 ohms.  The 1500 pushes 280 watts at 8 ohms per channel & with the crossover, it will eliminate some of the low end strain on the mains so you can get away with a slightly under rated amp.

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ARRRGH!

Never buy a 4 Ohm speaker!......ARRRGH!

Ok, I feel better........

Trust me Slick, it's ok to hit your single 4 Ohm sub with a bridged 1400 watt amp.....You can always turn the knobs down, you know?

Leave the tops on the 2500 running stereo full range-not biamped, just biamp into the sub.

How to biamp the sub?...It's Easy if it's got a built in crossover....Not sure if it does?, then simply unscrew the back jackplate and peer inside.....If it has builtin crossover, then feed it  a fullrange patch off tops amp and let it work for you....A built in crossover will absorb lots of extra amp power making the 1500 not so scary when bridged.

If no built in cross over, then patch in your dusty external xover and go two way where you send full range to the tops and just the lows to the sub......Where to cross?.....Start at 150Hz and maybe work down depending on your tops woofers.

You want to center 65Hz in the subs cause that is the freq the male chest cavity resonates at......for females, it in their butts as proven by the way they flip their skirts when the bass is right.

Bass is the hardest thing to get right, and it's also the narrowest freq range you will deal with, but for some reason it is the most expensive freq range too.........BUT, when it's right, it's worth it!

I totally agree there, your really limiting your future possibilities using a 4 ohm transducer.

The front load cabinet is appropriate for the size of most of the rooms you'll see these days. The bottom end will develope fast and not go by you and deafen the people 150 foot away at the back of the bar( like with a folded horn). I'm using two 18", 8 Ohm front loaded cabinets so when I bridge an amp and run them parallel i'm at 4 Ohms but moving twice the volume of air. I've experimented over the years from crossing over anywhere between 90 and 150 and have found that around 120 works well with my set up.( I use a pair of 15" JBL2225's in the mids)I'd rather have 12's in the mids but I'm just plain over run with 15 inch speakers. JBL's EV's Black Widows and Seleniums mostly.  So anyone out there got quality 12's and would rather have 15's let me know, we can talk

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Quickness of mind will deceive the eye

I am soooo glad i use my Furman Punch 10 bass in your face processor.....unfortunately....they don't make'em anymoore....

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Sweet Little Me In Karaokeland........

I'm trying to pencil this out without any of the equipment in front of me.  I'll need to do this:

Since there is no built in cross over, patch in your dusty external xover and go two way where you send full range to the tops and just the lows to the sub......Where to cross?.....Start at 150Hz and maybe work down depending on your tops woofers.

To go full range to the tops ... leave them set as they are now?  Which is right off the 2500, or do I need to pass this through the xover?

To send the lows to the sub, connect the xover from my mixer then to the 2500 with the 1500 off of it? Then send to the sub from the 1500?

Would the setup be any different if I had two of these subs, other than sending two feeds from the xover?

I would actually go with a frequency of 75-100 hz.  Also if you run the 1500 bridged, you are going to have to be more careful then if you swap the amps around.  That amp is too much for the sub in bridged mode.  1400 watts at 4 ohms & the speaker program rating (which is what you typically want your amp to push) is 700 watts.  The specs themselves say recommended amp power no higher than 700 watts.

Mixer to the crossover.  Highs to the top amp, low to the low (sub) amp.  You don't want the tops trying to reproduce the sub frequencies & kind of defeats the purpose of the sub in the first place, you could run into some phase cancellation, run it all through the crossover.

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Thanks Lonnie,

I might go and get an amp that will be better suited.  In the interim, if I only cranked the power knobs to halfway, would that be at 700 vs 1400 at full?

What if I employed two of these 18" passive subs?  Any changes?

Turning it down would be ok, but you wouldn't be getting the full efficiency of the amps potential.

If you went with 2 of those subs (same model), you would need a different amp - or swap the 1500/2500 for the mains & subs.  The 2500 would push both subs - per channel, not bridged.  Those amps will only handle a 8-4 ohm load bridged & if you tried to bridge both subs, you would be down to a 2 ohm load which would put a large strain & probable to blow the amp.

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Nope.........Disagree.

Those are not volume knobs!...They are only used to match input sensitivity when using multiple amps.....If you have two or more different amps, then you reduce the most sensitive amp as reguired to match the other so both amps react the same to the input signal.

Those knobs will not prevent amp from hitting full power-just control WHEN it hits full power.

Place the smaller amp on the sub bridged. Then reduce signal to the amp at the xover!...It's really simple and will work fine....Crank amp knobs to max and leave them there.

Leave 2500 on tops....Then patch from top amp mono(left signal) to sub amp(bridged).....Then crank tops to max normal listening level and then crank xover sub output till it sounds well blended and leave it there.....you have just limited max power to sub and done it safely at the xover-Now the sub amp will never see destructive input signal so leave amp cranked to max to allow max headroom.