@bruce19 That ’magic’ you’re looking for in an SET might be mostly distortion- they make quite a lot of it, mostly the 2nd harmonic which gives ’warmth’ to the sound.
I’d give your ST-35 another chance. Here are the considerations that will help it out:
1) driver/input tube- the NOS ones are better.
2) coupling caps. This is a bit tricky in the SCA-35 but in the ST35 I think you have the room to install something similar to what you see in a lot of SETs.
The 2A3 assumed the mantle of King from the 300b sometime in the late 1990s and the type 45 in the early 2000s succeeded it. Its not because any of these tubes sound any better than any of the others- its because the smaller tubes are easier to drive and its much easier to make an SET output transformer with bandwidth if you make it for lower power. So the 300b doesn’t stand a chance if you equalize all the other variables like power supply, transformer quality, parts quality and drive concerns.
But I’ve found that if you put a PP amp against an SET, the PP amp is obviously wider bandwidth and more transparent (easier to make out vocals and the like; because the higher distortion of SETs obscures detail).
So I think you’ll want to think this one through very carefully. Do lots of audition, but if you do so try to limit the variables! For example, most of the time PP amps are considerably higher power, use feedback and may not be class A (I think the ST35 is BTW...). If you really want to hear what the difference there is about its best done with a PP amp of the same power and build quality as the SET...
SETs tend to sound ’dynamic’ because of the way they make distortion; for this reason they really don’t have much more than about 20-25% usable power before distortion causes them to sound ’dynamic’. On this account you really need an efficient speaker so you never have to ask greater power! By comparison almost any PP tube amp has more like 90-95% usable power, even if its zero feedback.
I’d give your existing amp another chance before you change it out!