Studio monitors have a very flat frequency response. You're probably used to speakers that have a response hump (most likely in the midrange), so they "sound better". Of course, they're changing what the sound really is. Also, it may have been the way the speakers were setup. If you were listening off-axis, then the sound would also seem inferior from the monitors.
A reciever is a preamp and an amplifier in the same case. Since you have a mixer for your turntables already, I don't think a reciever should be necessary. You should be able to just run outputs right from your mixer to an amp. Or if your speakers are powered, then to the speakers. I'd still recommend some studio monitors if you can afford them. If you want to hear the details of what's going on with the stuff you're mixing without any enhancement, then they're the way to go.
For an amp, I'd look around at Guitar Center/ Musician's Friend. They have pretty good prices on stuff.