I have a small PA system though that is really great for home stuff or even a small venue, coffee shop, back yard pool parties and such. I have had it for almost two years and it is really nice, I use it for all kinds of stuff really and adding a couple of powered speakers makes it work great for a bigger venue or big outdoor stuff. I guess we see them a lot in my area because we have a couple of local dealers who carry them, it's a Kustom Profile One PA system and it might be just the ticket for saving your nice home theater system.
http://www.music123.com/Kustom-PA-Profile-One-PA-System-630506-i1153432.Music123
You won't get killer bas from the small speakers but adding a powered speaker or two will work out fine. The bigger system Two is a Stereo system and it has a subwoofer out capability but for your house this is really an awesome little system. The effects in it are great for vocals and acoustic instruments too. I really have got a lot of use out of mine and I seem to take it with me more than my other systems just because it's so easy to tote in the roller bag. It also has RCA in and out so it should be a snap to send your Karaoke audio in to the mixer.
Just a thought...
BSR