The problem here is that every amp reacts differently, even hearing this clip, it is hard to say. The other BIG thing I noticed was also a lack of volume. Anyway a tech is the best way to go. As a tech, here is what I would do before cracking open the amp.
1. Run a cable from FX send to FX return. These tend to fail in ALL amps. The jacks are switching, which means when you plug a cable in, it lifts the contacts from connections on the other side. Those tend to warp, or get corroded over time. Plugging a jack in and out only cleans the part that touches the jack, you may knock some crud off the internal connection, and make it work again temporarily. But I would replace them. They are cheap and easy to do.
Typically with serial FX loops, the sound is completely cut out when this happens. But some FX loops, even advertised serial ones, still let some sound through. Or maybe just enough is getting through. Hard to say. But running a cable clears this issue up fast, rules out the tubes or any other problem.
2. Run another preamp into the FX return/poweramp, or go from another heads FX send to this heads FX return. This will tell you if it is a poweramp or preamp problem.
If you still get nothing, then it could be the PI (Phase Inverter) Tube, which I have seen go bad. Preamp tubes tend to not wear out in a long time, it doesn't mean they don't fail though. Poweramp tubes or a leaking cap or cold soldering or blown transformer are also possibilities. Change the PI tube, you can swap ones from the other positions to test if you don't have spares, since we are only caring about the power amp.
If this doesn't fix it, take it to a tech BEFORE replacing power amp tubes. It could be an internal problem to the amp that is blowing tubes. So your new set could blow right away. I have learned this the hard way
3. If it is a pre-amp problem. To be extra sure, try running the pre into another power amp (FX send to FX return) Take a new or known good 12AX7, and swap in and out the different positions and see if that does it. If it doesn't, then take it to a Tech. From here it could be a leaky cap, cold solder joint, switching/relay problems, input jack problems. All I have seen lead to thin sound, no volume problems.
For Noobs:
Tube glow doesn't mean shit as far as functionality. Well for the most part...
There are high voltages in the amps that are stored in the caps which can KILL YOU even if the amp is unplugged. So don't poke around unless you know what you are doing.
If you are using heads to test the Pre/Poweramp stuff, make sure each head still has a speaker or load connected!
Don't swap tubes while the amp is on.
Tubes are hot! Typically won't cause serious burns. A rag or papertowel is recommended. For computer tubes, you don't even want to finger oil on there. Not so much with amps, but I will wipe them down anyway.
