Which best 5U4GB rectifier tube- Mesa Boogie or ??

Don't know specifically about the 5U4GB, but Mesa simply re-labels ordinary, current production stuff. Their 12AX7 is Chinese and the 5881's are Sovteks. Having listened to both of these "Mesa" tubes I guess that you would do better with NOS, even with a rectifier tube. FWIW.

I agree with Jim, go with NOS Sylvania and Tung Sol (harder to find). Don't bother with the Philips, you can do much better for low $$. Also, 5931 is a great military replacement for the GB, but kind of scarce. My amps use four GB's and I've found the Sylvania GB to be superior to even the earlier 5U4G by far. I'm stocked for life and could spare several if interested. It's the best place to start listening. I can give you an idea about upgrades later too. Had a pair of 300B mono's previously and did the experimenting with same results. Drop me a line. Paul, triode6sn7@hotmail.com

Look at the coke bottle Sylvania (shaped as it sounds). I have a National Union in my AirTight ATM300 SE amp and it is very nice, but hard to find. I have tried many. Rectifiers are relatively cheap off ebay, so you can experiment. Let us know what you find. Good luck.

jim, FWIW, The 12ax7's i last received from Mesa were absolutely identical physically and sound wise to EH's (they were relabled however to mesa logo). They do not sound similar to the chinese. Frankly thats not a recommendation for either.