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

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.

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

Rectifier tubes absolutely change the sound of your gear as much as small signal tubes and power tubes, IME.  I have rolled 5AR4 types in my Lampizator Golden Gate, and it can have a big influence.  Whether you will like the change a given tube is always the question....but it is another lever to pull in dialing in a system.

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.