"To my mind, the 5AR4 tube rectifiers were at least partly responsible for the preamp’s tonal integrity, fleshing out the lower octaves and, in particular, infusing upright bass and cello with a full complement of tonal heft."That's a bit of a sketchy claim. Its far more likely that the audio circuit itself is responsible for this. A tube preamp can easily do exactly the same thing with solid state rectifiers (for example, by using 6SN7s in the line stage). Seriously, solid state rectifiers are now good enough that they easily keep up with tube rectifiers in terms of noise, but they don't have the voltage drop (which varies with current draw) that tube rectifiers do. Its for this latter reason that tube rectifiers are preferred in tube guitar amps, since that can impart that 'blues' sound which is really the sound of an amp that has a sagging power supply.
If you were to measure the voltage drop across the rectifier, you would find that the best sounding ones for hifi are also the ones that have the lowest voltage drop. There's nothing magical here- they respond to physics just like everything else in audio. And again- the power supply has to be working properly in that the filter caps are good and sufficient to properly bypass the supply and prevent noise from getting into the audio circuit. It that bit is true, then the only way a tube rectifier can affect the sound is by the voltage it puts out. So it should follow that if you use modern solid state rectifiers, you can make it even better, and in practice that turns out to be true as well.
