About the rectifier question -- with some hesitation -- some people say there is an audible difference, whilst others scoff at the idea. I don't have the technical expertise, or audio experience to judge, but perhaps I'll offer a couple of observations of what I've read and heard. Many of the arguments against there being a discernible sonic difference are based on technical grounds, and not on the basis of experience. Of course, listening "tests" are fraught with expectation effects.
One argument is that the rectifier is not in the signal path, so it can't possibly affect sound. A counter argument is that the power supply is in series with the amplifier output, so it is very much a part of the sonic chain -- the point begin that the job of the amplifier is merely one of modulating the power from the power supply.