The UK distri had told me that Source and Pre react much more than power amps to mooking. In those days he had Gamut power amps. I didn't find them as transformative under my AR Ref 110. But then once I had a cheap Crown amp 500w on demo. It sounded bad, and on SP sounded unlistenable, and totally transformed to AR Ref 110 levels with mooks on.
Last week when I was in Mumbai an audiophile Jazzhead introduced me to wanted to try the mooks, he had a Wilson Alexandria X2S2 with Vivaldi Stack and Dan Agostino pre and Momentums and Opus MM2 cabling. System sounded great. He had it on Finite Elemente rack with some Stillpoints on it. Since due to rack space issues, we could not put Shun Mooks under pre and source, we decided to put one giant under one Dan mono, and an ultra under another mono. Not ideal. Also the monos being rectangular and slim, bunch up the front two resonators, which is not ideal either. Yet the transformation was pretty significant.
So, do they make a difference under power amps? Depends on the power amp. I will not buy a component that cannot rest on Mooks, e.g. anything that is slim, or too small to rest on a proper triangle, or one that has too many feet/screws under to allow it to rest properly on a set.
The Jadis JA 100 did damn well despite being slim.