I was in a Tweeter last week and the Pioneer Elite 54tx was at low volume driving some efficient speakers and it was screaming hot, also the 52tx above it was not playing although the power was on and it was screaming hot also. I have seen posts about HK receivers getting hot but this was too hot for my liking.
A lot of the heat comes from the power supply's losses. This is normal of many designs. Some amplifier designs also create a lot of heat even on low volume levels but provide great sound with low distortion. Put a little fan in the back of your stereo stand. Small computer fan about $5 with a small AC adapter works for me. mz