I have used the Philips 5R4GYS in my PrimaLuna ProLogue 3 with great success/results. The Philips tubes were purchased from Upscale Audio. They are the only tubes tried in this position other than the stock PrimaLuna 5AR4’s.
As
others have mentioned I would consider it a bad idea deviating from a 5AR4 type
tube, unless recommended by PrimaLuna/Upscale Audio.
From the PrimaLuna Tube Rolling Page:
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.