A couple of useful things to think about here.
The two things which really contribute to the H&K harshness/smoothness are speakers, (90%), and valves (<10%). If you are a classic rock, (I mean REAL classic rock from late 60s through to late 70s
), you are in exactly the same position as myself where you definitely don't want that top end fizziness that can sometimes sound good in higher gain playing. It could be your valves giving up but it is unlikely to be the preamp valves which last for many many years as they are under no strain whatsoever under any conditions. If you can put your hand onto a single spare 12AX7 you know to be good, I would try swapping them out in turn starting with V1. If you can genuinely hear a difference without ANY adjustments of ANY of the controls between the swap, then you may benefit. The power valves have a harder life but, thanks to the clever Standby that H&K implemented, they don't suffer the killing strain that traditional amps impose. I would try running with the Power Soak at reduced power to hear how they sound, then swap the outer and inner pairs and see if that helps. These two pairs always get different degrees of use if you use the PS a lot, (I use it all of the time). Again, touch NO controls between the swap, you must have the amp in exactly the same state to compare honestly. So, to the types of valve to go for....
My own view through long experience in electronics design and a lifetime of guitars and amps on top is that valves from different manufacturers make a less than important difference. To hear people talk you would expect that changing to a different make of valve makes a Peavey sound like a Fender. It can't! The effects are subtle at best and in a live setting of a rock band will rarely be heard in any meaningful way.
And there are some stupid practices accepted as normal by most people as the "musical myth machine" rolls on. Look into the valve ("tube") swapping thread in the GM36 forum, the advice is basically the same for the TMs. Go back to page 5, post 109 and look for the splurge I wrote about swapping for different valve models, 12AT7/12AU7/12AY7 etc. As a classic rock player you may decide at some point that you have too much gain on tap and you want to reduce it by choosing low gain valves. Those others are NOT equivalents to the 12AX7! They screw with aspects of the stage they are put into in ways hardly anyone really expects because guess what, most "experts" don't know of the consequences and just advise their acolytes who then become "experts" in turn to perform the sacred ritual and so it goes. If you feel you need to go down that gain reducing route use a 5751, it IS a decent equivalent and keeps the stage setup mostly as the designer intended.
As to different makes of valve being better, well as everyone says this is of course a very personal choice. I would say, consider this. If the info posted about peoples' findings on this is purely a matter of using their ears and picking which they think sounds best, then we would expect that there would be a pretty fair mix of opinions, as every make would suit some people's choice of music. So there should be some who actually prefer the sound of the cheap Chinese valves fitted as stock shouldn't there? Plain statistics. So where are those voices in the discussion saying they prefer the Chinese (or Russian) models to the "ultra extended crystal highs and caramel silky smooth mids" of the "better" valves? This whole area is riddled with "white line following". People say the valves make a real difference and that a particular more expensive type is better because they think they should feel that as others have said it too.
In my time I have conducted double blind listening tests on a number of aspects of amp circuitry, in particular on capacitor and opamp types. The "golden eared ones" can always tell the difference and describe it in glowing terms between them when they know which is which. I've never met anyone who could identify the difference when the listening comparison is instant, random and blind. No one!!! They don't know the real extent of the differences which can be measured between components and their impact on the circuitry. Valves are more difficult to set up in a test because as soon as there is a longer than a few seconds pause in changeover the brain takes over and conveniently forgets the first sound. The placebo effect kicks in and you hear what you expect to hear. And it absolutely has to be a change of a single aspect in the original unit, not two similar units side by side with different components in to compare. Those who are more questioning, (more engineering and less "mojo"), and honest admit there is not a lot of difference.
For my money, (and this is the line I take along with a lot of others here), I just use simple common or garden JJs for all my valve needs. This is based on the fact that they have never let me down in use. They also sound pretty much as I would want which is good. I have tested other makes and found very little if any difference, not much more than different valves of the same make and type off the production line. And I can get them virtually anywhere at a reasonable price. As an eye-watering shocker, go back to the GM36 valve swapping thread Page 5 and look at Post 122 onwards for a while. VoodooJeff is a professional musician and a good friend of the forum. He has access to Wathen valves. Have a look at their site for the prices and have some smelling salts handy. Jeff puts it very clearly in his posts from there onwards.
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