Atlantig is correct.
With a tube amp, I strongly recommend having a full change of tubes in a drawer. Tubes are very similar to incandescent lightbulbs, and about as reliable. At the very least, have two new 12AX7 ready to swap in.
How to diagnose:
Presume that the power tubes are fine. For the moment. They usually go out with a bang.
Is the sound affected on only one channel or both? Some preamp tubes are for only channel one or channel two, some are for both.
Are you using the FX loop? There is usually a tube dedicated to that.
Based on the answer to these, you can take a new 12AX7 and change out the likely problem tubes one at a time and see if you can find a bad one.
OR, just do the shotgun approach and change every one one at a time.
My experience is that the PI (phase inverter, the closest preamp tube to the power tubes) is most likely to go bad. So I always start there.
Usually Ch1 and 2 both go through V1. V2 is generally channel 2 only. V3 and V4 can be an odd assortment of functions including FX loop (both channels).
You COULD have 2 bad preamp tubes, but this is fairly unlikely.
It COULD be the power tubes, but usually you would either get nothing at all or it would be kind of OK, as the push-pull circuit actually works with one bad tube a lot of the time, just not quite as loud and awesome sounding.
Other causes: Bad solder joint on a plug. This can happen for a lot of reasons, the best one being from bumping or dropping the amp with a cable installed. Less likely with a new amp.