Where to put damping sheets?

Mburnstein that's a dumb answer. You don't want damp sheet on the mattress, they need to go in the dryer!

What results did you experience with the places you applied the sheets, and how did the placement relate to the power supplies?

The ulitmate result of dampening (to extremes) is that the music comes off as lifeless. I refer generally to overdampening the room. There are reflections in the recording that relate room ambiance where the recording was made. This isn't exactly what you are asking, but the idea is there.