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.

Elevick makes a fair comment. I agree with the adage "if it ain't broke, don't fix it." Sure, check the cartridge alignment.

However, I do caution that if the power tubes are old, they are susceptible to failure, which in some cases could take out bias resisters. I would still ask the prior owner the age of the tubes. If the hours exceed factory recommendations, I suggest replacement.

BTW, is the amp old?? There's a couple of threads running about cap failure, particularly electrolytics. Just a thought.

FWIW