Tube Failure question

Is the sound in both channels or just one? If it's in one channel try swapping the tubes from one channel to the other and see if the noise changes sides.  The power tubes are the large tubes yes and swap those they are most likely the culprit if it is in fact a tube or tubes going bad.

Pima Luna uses auto-biasing and runs their tubes at very low voltages. Tubes should last many years. Any given tube can always go bad of course, but with this being a fairly new amp I'm sure Prima Luna if not the selling dealer, would want to know about it. What did they say?

Yes the power tubes are the large ones. Probably EL34, about as warm and musical a tube as you could ever hope to find. Prima Luna are designed to run with a lot of different tubes though, so feel free to knock yourself out. My bet is after a lot of time and money you come back to where you started.

As far as power goes, that one is even more clear: No, you do not want as much as possible. Why? Granted one of the hardest things to learn, but watts just don't matter. What matters is how it sounds during the vast majority of the time you're not clipping. Which unless you have awfully inefficient speakers (which you said horns, so no way), or crank it really super loud, might be never.

If you want musical, upgrade to a tube preamp. If you want louder, upgrade by adding a swarm bass array (several subwoofers). But I would start by ditching the solid state.

Keep the Bryston preamp! I have one in use right now. Excellent SQ! Don't fall for the false belief that only tube circuits are "musical"!

Not true for vacuum tubes you get with stock amps are not verygood vs the originals from 1970 on down .the vacuum is of the gasses the inspection of the grids on average over 10 % variances inside gridsor more   years ago 2-3% was common 
the materials not even close maybe 3000 hours  on average NOS tubes from the 50s many times 10,000 hours , some rectifiers run for10 years . Emissions labs in Europe are great as well as a few others for premium brands . Call vacuum tube services, or Brent Jesse recordings to get true New  closely matched vacuum tubes a big upgrade in sound also..I grew up on all the classic tubes and the  majority of Chinese, and Russia tubes are average at best compared.

Quad of Tung Sol 7581A for around $100 will sort you out. Search this forum for more info.