I will say, for a few years I used a Sherbourn 5/1500A power amp(200 watts per channel) for my front L/R channels in my bedroom system. My AVR is an Onkyo TX-NR809, which is a pretty ballsy AVR. However, it was obvious that with the Sherbourn in use, there was more bass slam and fullness than with just the Onkyo. Interestingly, however, it seemed that the initial attack on bass drum shots was more immediate and better defined with just the Onkyo, while the impact was "fatter" and less defined with the Sherbourn.
But the biggest issue with the Sherbourn is that either it, or the interconnect cables, smeared the mids and highs, especially high vocals. Vocals and instruments were just not as clear with the Sherbourn in place. And it seemed like I was losing some of the musical overtones, which took a bit of life out of music. I ended up preferring and going back to just using the 809 alone. If you do want to try a separate power amp, I would suggest making sure you get an in-home trial period and thoroughly compare the sound with and without the external amp.