I've been reading many threads about adding an amp to a receiver and there seemed to be lots of different opinions whether it's worthwhile...it may increase sound quality but not necessarily...you need a preamp, etc.
I bought a used Adcom GFA-535 for $160 and connected it to my Pioneer Elite VSX-21TXH AVR and Polk RTI A1 speakers. The results were astounding! This 60wpc amp obviously puts out more/cleaner power than my $800 "110W x 7 all channels driven" receiver. I clearly noticed more clarity and detail and the speakers seemed like they opened up. Was really a dramatic increase in sound quality. Before buying the Adcom my friend brought over his Adcom GFA-555 amp and GFP-565 preamp and hooked it up, which is what led to me buy the GFA-535. The sound was very similar. Only difference was with the 555 and 565 preamp the bass sounded fuller in full range with no sub. Not sure if this was from the 555 or the preamp.
So from my experience if you are just using a receiver you are doing yourself a disservice.
It really depends on the speakers. I use a separate Adcom 545II to drive my JBL L-100's and I can tell you that they sound much better than just using my Yamaha AVR. Some speakers really blossom with more (clean) juice.