The adage that "you are listening to your equipment's power supply" applies. Rectifiers make differences that are easily heard. Which means you have no objective idea which is real. Like lots of other things in hifi, especially tube gear, tube rolling is an exercise in finding the right "fixed-parametric equalizer" for you. It goes beyond EQ because dynamics and dynamic elasticity are also clearly affected.
For all the reverence for NOS Mullard GZ34, 4 or 7 notch according to one's theology on the matter, in many of today's circuits they can make an amp sound "tight-sphincter-ed and unengaging." They are great (and durable) rectifiers but in many circuits can lack musical elasticity. Among new production GZ34, the JJ is the most elastic, but you just have to change them out on a schedule. For daily use stuff, I change them on schedule every 9 months. Some of my amps that were supplied with GZ34 don't use the full capacity of the tube, and I can instead use NOS 5v4 or 5u4 rectifiers. When that works, the vintage 5v4 usually sounds sublime.
One inexpensive modern production rectifier that I love in some of my preamps, power amps and guitar amps is the STR (Special Tube Request) version of the Shuguang 274B that thetubestore.com sells as part of their Preferred Series. In this case, the STR is ultra-hard vacuum, thicker glass and a higher grade plate plating. It's not a true 274B, having the lower filament current draw of a GZ34/5ar4, but a voltage drop about halfway between a true 274B and a true GZ34. It sounds disciplined yet elastic at the same time, with excellent, hard-hitting base and quick, crisp transients and highs. It's proven long-lasting for me. Really nice $35 rectifier. Beats all the Russian rectifiers I've tried in the past, and betters a surprising number of NOS variants.
The real point here is that rectifiers can have a more profound effect on amp sound than rolling the output or input/driver tubes. So it's a good idea to start there, and it tends to be less expensive that beginning with tube-rolling quads of triode, tetrode and pentode power quads.
Phil
