Studio reference monitors are not PA Speakers. They act more like High-Fi speakers, but just hook up to PA equipment. They are designed with the idea that you will be running your signal chain through a series of filters, like compressors, effects processors, etc. Basically, they are similar to home theater speakers in that they are designed to reproduce produced, controlled program sources.
If you just hook studio monitors up to a PA system and sing karaoke through them without the intermittent filters, you are going to notice that they can not reproduce power spikes very well, such as high volumes and vocals. They expect the compressor and other filters to take these spikes out (or for the program material to be already produced, as with the stuff home theater speakers are designed for), so the speakers are not designed to reproduce them.
So, if you plan on using these speakers at higher volumes (like when you wanna rock out!!), they are not going to sound very good. You will need to keep your singing volume and the volume of the music at nominal levels (like the standard volume you would listen at in the car) for the speakers to work as intended. Absolutely no screaming into the mic. It will blow these types of speakers very quickly. Also, the bass is going to be virtually non-existant due to the small woofers.
So, you can do what you want to do, but the CORRECT answer to your question is DON'T BUY HOME THEATER SPEAKERS OR STUDIO MONITORS FOR KARAOKE USE!!
You need a PA system!! If it were me, I'd go with some 12-15" PA speakers that are designed for live performance like karaoke, though this may be out of your budget range.
Here are all of AMS's complete PA systems under $500:
http://www.americanmusical.com/sort.asp ... 06_77_1001
I would recommend the Sqiuer 4 ($230) if you want to stay within the same range as those other speakers, or the Fender Passport 150 ($400) if you want something that is going to sound really good for the $$ (Squier is Fender's discount brand, the Fender is a much better unit).