My experience with using amps on isolation transformers is that they do suck a bit of life out of the music. Lack of attack and smoothed over sound. I've tried probably a dozen power conditioners with amps, none have worked thus far.
I did use Bybees in my former amps, at that time I didn't detect any downsides, couldn't really tell you if they affected RFI as that problem had already been resolved. I do have some Bybees ready to install in my present amp, just haven't gotten around to it.