BDP24,
The 6DJ8 tubes come in a large number of variants with different names for the US spec, Euro spec, and even Russian. These variants often come in 3 versions: a consumer (some can be really good), commercial (usually very good, but some can be AMAZING, and military spec (which can be better still. The commercial and Mil Spec tubes cost more, but you will get longer life out of them than the consumer grade; and they sound sublime.
I stay away from tubes made in Asian countries (too hit or miss), most tubes from the 1950s and 1960 are incredible, 1970s are really solid if not great, and some 1980s tubes can be really good.....but rule of thumb, older is better.
Some of my favorites are:
(1) Inexpensive - RCA "Clear top" 12AU7 - There are better tubes, but this is a steal at $20 a pop and easy to find in matched pairs, same with similar Russian tubes Voskhod 6n23p
(2) Next level - Better sound, higher costs. RCA "long black plates",1950s and 1960s, GE 5814 early 1960s, Amperex ECC88/6DJ8 Made in Holland", Philips 5963 (Mil Spec)
(3) "Damn these are good!) - Amperex 6922 "PQ" made in Holland, Amperex 7308 "PQ" made in Holland, Mullard 12AU7/ECC82, Mazda 12AU7 made in France, and Vokshod 6N23P "single wire) made in Russia
(4) Holy Grail and ungodly expensive - CCa made in Germany (mostly), 12AU7/6DJ8 "Pinch Waist" version, some people will even put the hard to find Vokshod 6N23P "Single Wire, Silver Shield" here as well
Advice: Read through the link I will post below, and use a reputable supplier that will know exactly what you are looking for. I have used Upscale Audio for rectifiers, but prefer Tube Depot over them. The best in my opinion is Brent Jessee out of Illinois......knows everything about tubes, carries a large selection.....even really rare tubes, and is a really honest guy.