so you had no problem using the 5R4GY instead of the 5AR4 / GZ34 on the Prologue 3.
Have you used them for a long time?
What improvements did you perceive?
From the PrimaLuna website...
"The 5AR4’s used in our preamplifiers are considered to be fantastic. It is as close as you are going to get to a classic Mullard. We have found that the Mullard and Gold Lion Russian re-issues are not as dependable in PrimaLuna preamps, so it’s best to stay stock when it comes to current production 5AR4’s. Do NOT use 5V4/GZ32 as they may fail and cause damage not covered by warranty.
There is a vintage tube that is quite popular called a Philips 5R4GYS. Very cool looking tube that lasts a long time. It has a slightly warmer presentation, if you need that. Read the reviews from other users. A lot of people like it, but if your speakers don’t have enough top end, it may not be a good idea.".
I do not know what to think. They expressly advise the replacement of the 5AR4 / GZ34 with the 5R4GY.