I added a two channel amp (ATI 1502, which is 150 wpc) to my RX-V2500, and I personally thought the entire sound got better. This may be due to there being more power available for the center, surrounds, and prescence speakers, or it might have just been my ego telling my I didn't blow an extra $ 400 for nothing.
if you run the YPAO or do a manual run with the test tones, you can balance the sound out (as long as you remember to turn the amp on before you start the test

)
The system does run quite a bit cooler with the additional amp. If I had it to do all over again, I would probably go with a three-channel amp or three monoblocks for all three fronts (left, center, right) instead of just the two.