The question often comes up, I have a ______ amp, what tubes are best to make this thing sound killer? This can be a really simple question that gets very complicated so I will try and break some portion of it down for you.
First, the most important factor with tubes, preamp or power amp, is how consistently they conform to design specifications. This is why companies like TAD (Tube Amp Doctor) have been so successful even though many of their tubes are a re-brand of models already on the market.They have a really good testing procedure to ensure consistent and reliable performance of their tubes. Anything that doesn't meet their specs is tossed and going through an extremely large volume of tubes is necessary and allows to have a more narrow range of acceptable performance. Same goes for for Ruby Tubes and some online tube retailers.
If you're already noticing a slight bias as to what brand I would recommend on the current market, you'd probably be correct. Companies like Groove Tubes, which essentially started out the same way, have failed to keep up in the consistency department and I'm not sure if it's just laziness, relaxing their testing parameter margins or if they are completely unaware of the high failure rate. And you will notice for short periods, factory testing of a "new" tube model will be really good and within a few years declines. Reasons for this can be anything from machinery wear to getting a bad supply of raw materials.
From what I can gather, there are only 3 or 4 factories in the world setup to manufacture all the current production tubes on the market. Even the TAD STR tubes are current production tubes with some slight, and mostly cosmetic, variations. Basically what that means is, find a reputable supplier that tests and matches tubes themselves.
To bring it back around, there are so many different aspects as to what 'the right tubes are for your amp that this article would be way too long. I intend this to be an ongoing discussion, so I will hit on a few of the high points for now and focus in on other key points in some upcoming posts.
There are slight tonal variations between tubes, slight gain factor differences and electrical tolerances that make them suited for different applications. If you want this to be a short read, a well matched set of Sovtek 12ax7WA's and 5881WXT's will out perform poorly spec'd "premium" brand tubes. If you have a little more to spend, I'd highly recommend the TAD sets as you will be sure to get top notch, reliable tubes of whatever model you select.
Assuming that the tubes you are dealing with are properly sorted and meet spec within a reasonable tolerance, the way I divide it up is;
1. Tonal profile of top end and upper mids of the tube. Does it have slightly more top end, how much air is on the top and what is the upper mid-range like. For preamp, the Tung-Sol 12ax7 and the Chinese 12ax7 fit in this class of more top end and articulate upper mids with a little higher gain. Something like the JJECC83 tends to have less highs but a very pronounced upper mid-range. I would also narrowly put the Mullard 12ax7 CV4004 here as well for more top end. For Power tubes there seems to be a little less variation and I like to look for overall clarity, articulation and just good amplification since your tone has already been mostly shaped through the preamp stages. Properly setup in your amp (and assuming again the tubes have passed a good test procedure) power tubes are meant to give the final output amplification and have a flatter response. You would likely not be that aware of differences in a proper set of matched power tubes across brands except in an immediate back to back test and those differences would be subtle. Again, those difference would mostly be due to variations in spec from tube to tube. With that said, I find JJ's to be the least articulate and clear, some of the Sovtek's as well. But any of these can still perform very well.
2. The flatter response or more balanced preamp tubes. This is pretty simple, yes you can and should have flatter response preamp tubes in certain stages, it will benefit your tone. My go to is the TAD ECC83WA but could also include the Sovtek 12ax7wa/b/c or their LPS. From what I gather, the TAD ECC83WA is actually a Chinese copy of the Sovtek 12ax7WC but with the rigorous testing procedures of TAD applied. The TAD 12ax7S is also a really good choice, actually my favorite tube on the market right now. And if you like that one take a look at TheTubeStore Preferred 7025. The thing to be careful of is tube swaps like the 5751 which has a pronounced mid-scoop. In older Fender amps that already have a large mid-scoop, do you really need more of that? Some amps can benefit from it but it depends on how you are running your rig.
3. High gain vs low gain. High gain is not always the answer. I can already hear the clack of keyboards with the vitriolic hate I will receive in this area. I will write a whole post about this in the future but getting the highest gain tubes in every position could be the biggest mistake or misinformed action that I see. For now, the abbreviated explanation is, in guitar amps, more gain = more compression. Compression = less dynamics both tonally and in volume and articulation. Now, having 1 or 2 higher gain tubes in select positions can be very useful but that varies by amp. Usually I like a normal spec flat response tube in the V1 spot. The way I approach it is, lets get that great guitar you've spent thousands on, amplified without a bunch of fizzy nonsense and compressing all the flavor out of the sound, before we start seasoning it to accent it's unique points. But, again it depends on what you are going for.
Where does that leave us? Don't believe everything you read in articles about your heroes or on forum posts and think getting those parts will yield a result that's good for you. It's an ever changing market where rumor and fact mingle . The sheer number of different brands and models of tubes on the market are mostly for marketing. The most important factor in picking any tube or retailer is a consistent reliable source with a track record providing tubes that meet specification. Well, and actually changing your tubes on a regular basis. A well spec'd set of Sovtek 12AX7WA's will kill a poorly spec'd set of any "premium" brand or a worn set that have been in an amp treated like a farm animal for the last 4 years. Tonal variations are subtle and only relevant for seasoning because it's the way your amp circuit is designed and power being supplied that has the greatest effect on the tone and feel. If you're ready to explore the subtleties, experiment away! Most any amp will sound great with fresh tubes as opposed to old.