first off find a high end dealer that can "loan" you a few amp's to try in your system. I didn't see where you mentioned your budget, nor what you wish to improve. Here on Audiogon, The Music Room, etc. you can find a variety of used power amplifiers (two or three or five or seven channels) that can add to your listening experience. Do your front speakers allow for bi-wiring/bi-amping? What is their impedence curve? Most A/V receivers don't double the power into lower ohms, so if the speakers go into lower 6 ohm or 4 ohm territory, a power amp that doubles from 8 to 4 ohms will definitely sound better than the current setup. There are many variables, but if you are looking for "oomph" and bass slam I personally can recommend Krell & Aragon. Krell make some nice A/V style amps that are less in the A/AB power output than their bigger amps, but can still drive lower impedence speakers. Have fun looking for the next improvement to you system, and be sure to listen to it with your normal music.
Moving to separates
Does anyone have an experience they'd like to share about going from just an AVR or integrated to using external amp(s)? My Integra AVR has 100 W per channel (class AB), all channels driven, and sounds very good, so it is not clear to me that there is any advantage to connecting its preouts to a separate amp. Might be a total waste of money. Please mention your specific equipment. Thanks!