How much does tube brand REALLY affect sound in terms of rectifier tubes?

Seems someone could manufacture a low impedance/low voltage drop valve rectifier(now), that provided sonics at least as good, as those made over three decades ago. The cost of even the best, most desirable, NOS rectifiers would drop like a rock.

There are a few companies now manufacturing tubes, that have tried to produce valves, copying the internal construction/design parameters of some of the most popular, from decades back. Mainly, what they’ve succeeded in duplicating(capitalizing on) is the marque only(ie: Telefunken/Mullard/Gold Lion/Tung Sol), as none genuinely perform like the originals. The time involved in hand construction, attention to detail and(in many cases) materials used, are no longer either profitable or(perhaps) available.Try some authentic, older, well reviewed valves(as mentioned above). Worst that can happen is you’ll have to turn them, should they not turn you on. It may cost a few bucks to experiment, but- what doesn’t, in the audio affliction/addiction? Our, "rewards" are SO subjective! Happy listening!

Voltage sag and compression aren’t the only cause of changes in the sound of a component's presentation.  Mainly, that affects it’s dynamics. That’s why I said, "sometimes", that will improve with a rectifier change. The short answer to your question is, "yes". BUT- there’s much more involved than just a, "different brand" involved. Most of us that have enjoyed various aural improvements through rectifier upgrades, have been using higher quality, NOS pieces, manufactured decades ago. Given that relatively few newer components have been designed with valve rectifiers(and/or regulators), there’s not been much demand and little motivation to produce great ones, these past 30 years or so.

Another thought: Given you have monoblocks, you could try installing a GZ34 in one channel’s amp, running the system mono, and compare the sound of the amps, before buying a second. Everyone listens/hears/judges differently.

There’s some good info regarding 5AR4 and GZ34 tubes here: (http://www.audiotubes.com/audtube.htm)

If I still owned anything that required the 5AR4/GZ34 rectifier and hadn’t changed them already, I’d be investing in either a Mullard(Blackburn) GZ34 or Philips 5R4GYS(if it fits). For over four decades, every piece of gear that I’ve owned(plus a plethora of my customer’s and friend’s pieces) have had their(generally inexpensive) rectifiers replaced with faster and softer recovery diodes/bridges(when SS), or NOS Mullards(when valved), as the first upgrade performed. Without exception, there’s been less background noise/hash and a greater sense of ease, realism,(what I call), "organics", and(sometimes) more dynamics. You’re listening to your power supply, and the better each of it’s components, the better the overall presentation. I’d probably have the Amperex tested, and if strong, leave it alone. They made some of the very best rectifiers out there. (https://www.upscaleaudio.com/collections/vacuum-tubes/products/philips-5r4gys-made-in-holland) (http://tctubes.com/5AR4-GZ34-tubes.aspx) Given the usual lifespan of Mullard’s rectifiers, one may outlive you and whatever it’s powering.