There's tons of threads on "which is the best GZ34". Just search around this forum, AudioKarma, diyAudio, and Audio Asylum.
Groove Tubes are all re-labels. Who knows where they bought them. Could be Russian, could be Chinese. They're not American made - as far as I know, no US manufacturer still produces the 5AR4.
Older (1995~2005?) Chinese rectifier tubes have a bad reputation, even worse than the JJ. They're known as tiny fireworks displays in a bottle. On the other hand, recent reports suggest the latest batches of Chinese GZ34 are actually quite good. I have a pair of "Ruby" labeled Chinese rectifiers, and they work great in my Mark III.
Sovtek seems to make a consistent 5AR4, but they're seldom noted to be outstanding. I have a Sovtek 5AR4 which I've used in light duty applications (amps which drive a single pair of 6L6 types, or a quad of EL84) but I don't know how well it would hold up in a Stereo 70. Keep the first cap small, and maybe it'll be OK.
If you want a really good rectifier, you probably need to find an old (but strong) Blackburn Mullard. They're identified by hole in the key pin and the distinctive Philips codes printed in grey paint near the base of the tube. The Japanese versions (Matsushita? Hitachi?) with the cross shaped seams in the top are good too. Just be warned they will cost you $50~$100, and you'll probably have to hunt one down on eBay. Occasionally you'll get lucky and grab one for cheap, usually when the seller doesn't really know what he's selling.
The old US rectifiers (GE, Sylvania) aren't bad either. Not quite as good as the Mullards, but usually better than today's stuff.
Here's a tip that will make any vacuum tube rectifier have a better chance of lasting longer: Install a pair of solid state diodes between the plates of the rectifier tube and the leads of the power transformer. You can use the spare pins on the rectifier socket as tie points. There is a good thread around here which mentions the idea, and it has a nice photo in it which clearly illustrates the wiring.
edit: Here's a good thread:
http://www.diyaudio.com/forums/tubes-va ... stion.html
Look at Trout's diagram in post #12, and note Kevin's comment in post #13.
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."