This link will keep you busy for awhile. Don't go crazy trying to find the "perfect" tube. Many factors are involved...
You may find some interesting reading here: Joe's Tube Lore.
My personal favorite is the Amperex 7308 PQ white label, but it is all system synergy and personal taste dependent. I know others who love the Mullard sound. There is no one right answer for you, just as no one can tell you which interconnect you'll prefer. There is no "Best", you'll just have to figure it out for yourself.
If you are looking for new production tubes (cheap) try JJ's E88CC'S (6922's). They are a fairly warm tube and fairly reliable. The EH 6922 is probably between the Sovtek (which is about as 'cool' as they get) and the JJ tone wise.
I like (use) a couple of NOS's the cheapest of which is the EI 6DJ8 (if you can find any), Tesla E88CC's, Amperex Bugleboys , and Seimans 7308's, but high quaility new, unused versions, are hard to get, expensive, and as John sez, for you they may not be 'best'.
New tubes take between 75-100hrs. to fully break-in. and even after that the sovteks are still not going to sound that good. If money is not an issue, nos tubes will sound the best. Particularly amperex 6dj8's made in holland in the early to mid 1960's and of course the siemens-halski are suppose to be the best of the best. Unfortunately, the prices are very high(anywhere from $200-$300 pr.)sometimes you can find for $150 pr.
As was mentioned before the electro harmonix's are not bad for current production tubes also, the 6h23 russian tubes are not bad for the money either($27.95 a piece) but for my money the matsushita 6dj8's are the best of the lot after the amperex, siemen's mullard's and they can be had for $30 a piece from the the Tubemonger.
disagree with post about tube break in.....IMO there's nothing to break in....tubes only start deteriorating the moment you use them until they eventually fail. For NOS 6DJ8 I liked orange label Holland Amperex.....full bodied sound with good detail. Cheaper alternative is cryo EH 6922, but thats not a 6DJ8 but is replacement like the 7308.
Firstly- Extirpate the Sovteks post haste. They are bad from the start and you are correct never improve. Despite Joe's Lore, my British tube Guru told me to try the CV 2493, because it is the only tube in the family that doesn't sound blike any other it is better. Not even it's more common little brother the CV2492 sounds as good. If you nlike Siemens then chan chances are good that you will like these. They are Military Mullards but don't sound like a Mullard at all.
Upscale has them in stock which is a bit odd. They had not been seen for a long while. The picture shown is unlike any I had seen before. I found them eventually from "Tango Tubes" of Sweden. As probably NOS but they did a great job imitating an NOS tube NIB, properly faded original packing cigar roll paper etc. matched and a royally expensive of course.
I have no idea if they still have these relabelled cheap Sino tubes made to look exactly like the real ones snd even managed to sound just the same as the real ones as wewll. As I said they don't sound like any other 6922 in fact they were given a special ID code E88CC-01.
Colomor Electronics (UK) has an Ebay Store and still lists "grey pin E188CC 7308" tubes at $40/pair. Claim they were surplus military stock for Polaris subs. I bought a few pairs a while back and, while the acid etched code indicates 6DJ8 and not 7308 (and 7308s s/b gold pinned), they are Amperex Holland manufacture. Given the insane prices asked for vintage 6DJ8 family tubes, these may be a bargain. I've not used mine as I've moved on from gear with such tubes, but may sell them of in ten years to help fund retirement.
I've tried Philips tubes, JJ, Sovtek and RCA. Now I have to say that the differences TO MY EARS (disclaimer: personal experience, no universal truth) are fairly small. I was lucky enough to stumble upon a Philips E288CC SQ. That is indeed the best sounding tube I have, but the difference with the standard E88CC from Philips is subtle. So don't start spending fortunes on very rare tubes - first try out the cheaper ones so you get a feeling of what you can achieve with tube rolling.
Go over to Audiocircle.com and check out electronic engineer Roger Modjeski's exposition about 6DJ8/6922 differences in the Music Reference section of postings. Basically, there is not any substantive difference. Depending on manufacturing tolerances, a batch of tubes can be labeled either 6DJ8 or 6922. Aural differences exist within this family of tubes of course, but don't focus on 6922/6dj8 as a factor of sonic differentiation.
Hi already have a Carver SDa490t in mint condition, I buy it in a dealer new one in 1992. Well I just fin a pair of Philips E88CC SQ special quality, not to expensive, with this tubes your Carver run magical ok, deep bass, excelents mids and extended highs, is just a dream, I put some acoustic parts inside the Carver and a nice foots, please put some rubbers on the tubes.
I run a Njoeb Toeb CD4000 too, but with this tubes the Carver it's much much better
I have not...and as is the case, most things audio, it's controversial. No one (too broad) very few deny that certain tools last longer, and that there's a molecular shift in products that have been cryo'd, but many technically oriented (note, I didn't say whether they're experts, as there's no way of knowing)folks openly scoff at the basic concept.
So, have you tried cryo'd G"
As to 'best'...again, a quick story.
A previous customer, from 1985, called and wanted to hear the LSA system--that is the LSA Statement amp and LSA2 speakers.
Other than the room being 'ugly', I had set the speakers up carefully, yada, yada, and was ready for him.
He's a very good listener, so I knew, for him to be interested, it had to be right, (but mostly, I can't stand it otherwise).
So after listening to a VERY wide range of music, I realized that I had my 'every day tubes' in the unit and not my Amperex. By way of explanation, my audio system doubles as a video--movie watching etc. when I do TV stuff, or again movies. So I keep the factory tubes in, and they're acceptable, expecially for that kind of listening.
I keep my 6dj8, Amprex, circa 8/62 (wow, 48 years old) handy, but FORGOT TO SWITCH! Duh.
So, I said, 'Stop the presses Doc', let me change something. I put the other tubes in, (takes less than a minute), and ice cold, he said "OMG, that's amazing." "What was virtually hard and stark sounding, now sounds rich and much more harmonically correct!"
Of course, he was spot on, which is why I love those tubes.
So...anyone who is more than a casual listener should appreciate them.