There was a recent thread about this. I believe that they can be used.

I can confirm that I am successfully using a Philips 5R4GYS Made in Holland from Upscale Audio in my Super Zen with no problems. I am not sure how that compares to a 5R4GY.

I emailed Steve/Sarah about it a while ago before purchasing. I may have posted about it before. It sounds good, but I am not a review writer.

I also use a preferred series 274B from the tube store dot com without any issues.

Finally, I also use a TAD GZ34 from cryoset dot com. It works well.

i know its kinda late ,but I tried the 5r4gya by GE, and it maybe the second best sounding rectifier behind the 274B that comes with the super.