How do you know when to replace a tube?

Sumhull, As a threshold matter, I assume that the rest of your components are matched well and are in good working order. Just focusing on the tubes, 50 hours by itself is nothing. The real question is how many hours are on the tubes from prior use. I would try contacting the prior owner to learn how many hours he/she put on the tubes.

Alternatively, if a retube is not too expensive, just replace them -- the whole lot. Aside from sound degradation, old power tubes are more susceptible to arcing. By itself, not a big deal unless they take out a bias resister with it. Been there and done that one.

Last comment is I would call the factory to learn what it recommends about tube age and recommended replacement intervals. If you still have a problem, it might relate to electronics which is beyond the scope of your OP.

I'd replace the tubes if you can, have the old ones checked to keep for replacements. I had Quartets for years, and they are some of the best sounding (if not the best) Klipsch ever made. Good luck.

old tubes sound boring and make you want to sleep,

new tubes will sound vibrant and full of life.

That amp should never sound bright, even with old tubes

IMO of course...

.

They do gradually degrade but can also just quit working.

That would not be the problem with the sound changing from beginning to the end of the album. That can be caused by improper cartridge alignment.

I'm with the others; buy a new set of tubes and see if the sound improves.