I agree with Ticks77. I have a Yammy 5760, and when I use the pre-out with my 75x2 @ 8ohm amp, it sounds much cleaner, and the high's sound better. lol actually everything sounds better, high's, mid's, and low's.
What I would do is, get one of the following receivers, and see how it sounds:
Denon:
AVR-2808CI ~$799
Onkyo
TX-SR805 ~$700
TX-SR705 ~$600
Pioneer:
VSX-1018TXH-K ~$599 MSRP
If it is still lacking either buy new speakers, or buy a external amp, and use the receivers pre-outs.
Or I guess, you could just go to ebay, buy a $100 pro amp and use your existing receiver. Some of those amps though have loud fan's, you'd have to do a little research.
Good luck.