Clearly there's something to be said for the what it goes into and what it "ought" to sound like but that said and although I'm a devotee of old glass...
In a newer tweed application I've kind of liked the Golden Dragon PRC produced, the PRC made T.A.D.s as mentioned above - both having a fairly nice "glassy" edge, and the increasingly pricey (older) Svetlana produced Russian Winged Cs. I also had good luck with the GT screened 6L6GEs. I found them a bit less punchy than I normally like in a BF circuit but they sounded pretty good in a reissue LP Twin - they also a decent life span (as did the others).
I too am an old glass devotee, but I have used the JJ and Tung Sol 6L6GC-STR with good success. That is until a bad batch of JJ 6L6CGs came down the pike late-winter and through the spring of this year. With all the good comments I've seen from the folks here who know, I want to try the TADs to see what all the fuss is about.
Depending on the amp, one of those three should work just fine, provided you buy from a vendor that screens and matches them before pushin'em out the door.
I tried a number of them including the much touted winged C, but even tho i never really liked JJ's much in other tube types, i gotta say the JJ blew away most i tried and easily whipped the revered winged C. I have never found any JJ besides the 6L6 i liked so much, and the great ting about it is they are as cheap as anything, plentiful, and JJ's have been around for a long long time and they never drop tubes or go out of business like so many others. Can't recommend them any more highly.
I'm a big JJ fan as well. They have been solid and dependable for me. I do have a pair of =C='s in my BF Bandmaster that really improved its sound, but that's in comparison with a poorly matched pair of Sovtek WGCs that they replaced. They sure have gotten pricey, though. I recently put a pair of Tung Sol reissues (Russian) in one of my '65 BF Tremolux's. They sound pretty good to me.
My big complaint with JJ power tubes has always been their venerability to rattles. More often than not the start sounding like a bunch of bb's in a peanut butter jar in a very short time.
Now that is party because I used them in combo amps but at least some of the other companies use a spring like centering tabs to limit that kind of thing.
The JJ's sound as average as most of the other current production otherwise.
The TAD 6L6 is a decent tube, but it's Chinese. You can get them from GT, Ruby, or any other re brander. As mentioned above most tubes are imported from just a few plants around the globe. 2 makers China, 2 or 3 in Russia, and one in Slovakia. Retailers like TAD, ARS, Groove Tube, Ruby, do not actually make tubes, they buy massive quantities with their names printed them. New Sensor corp owns SovTek, EH, TungSol. They actually produce their own tubes at the Reflector factory in Russia.
The tube you feel is "best" depends on personal preference. I have found the JJs to be the most durable electrically, but they do tend to suffer mechanical breakdown in combo amps. The bane of vacuum tubes is microphonics or mechanical noise. The most common cause is from the mica supports breaking down where the contact the glass envelope. Combo amps are horrible on tubes - they rattle them to death. Once the mica spacers start to rattle, the problem just grows and grows until it's intolerable. Svetlana - Winged C uses stainless steel springs between the glass and the mica, but these can get noisy too. I think the tube that seems to have the best record against mica breakdown is the SovTek 5881. They are a reliable tube - but not the sound for everyone.
As for as NOS US and European power tubes, if you are willing to pay obscene prices on a consumable part - that's you business. In many cases, the new power tubes are far better than the NOS counterparts. I reserve my stash of NOS power tubes for my HI-FI gear, and selling them on Ebay to support my amp collection. I would never dream of using them in a guitar amp. An exception of this is my Univox, which uses 6973 tubes, which are nearly extinct. The new ones are no where close to correct.
The TAD WGC is an excellent sounding, long-lived tube in my experience and my preferred new production 6L6. I know they're made-in-China private labeled tubes. No idea if they are identical to any other currently available Chinese-made tube from any other rebrander.