SET owners, need help with Rectifier tube


I am looking to replace the rectifier tubes in my Coincident Frankenstein monoblocks. The current tube which came with the amps is a GZ4P. I have learned the P designation indicates a Chinese tube.

Does anyone have experience with a Russian substitute which is a 5U4C?

Mullard has a substitute which is a GZ30 which go for $50.00 each. Is it possible these tubes can effect a sonic improvement?

Any input would be much appreciated. Thanks.
montepilot
I have found Mullard GZ30's on ebay for as little as $22 per tube, they sound great, closer to the 5y3 than the 5u4 in voltage drop. However, they don't have the high voltage/current limits of the 5U4 or the 5AR4, you would have to be careful about using them in an SET power amplifier without knowing they are capable of the output, I would stick with the other types for power amps.
Third on calling Kevin at upscale, and I also agree on the Mullard rectifier(though Kevin has none listed for your amps at the moment). I've found them to affect the most improvement, and last the longest also(Blackburn plant).