I second/third the microphonic tube assessment as well.
Combo amps are particularly subjected to LOTS of shaking, rattling, and banging around! This is hard on the amps internal parts and the tubes. When you are playing at performance level, the chassis is vibrating like crazy and the tubes vibrate too. The rattling ( rice shaker ) sound you are hearing is likely a tube having its guts shaken, and you can generally sus out a microphonic tube by tapping it with an implement of some sort while the amp is on. If you can hear the tink, or the subsequent disruption of the tube's internals when you tap it, the tube is microphonic.
As to the low volume, it is possibly a bad phase inverter tube, but any tube that is directly in circuit that has drifted in performance can also cause low output. V1 isn't always my first suspect if the volume is low because you would then also notice a lack of distortion from the preamp. I.E. you have to crank the gain up a lot to get any real distortion. Since each gain stage will buffer the last one at the very least, the most common low volume problem is from the PI tube not driving the power tubes hot enough. It may also just be that the power tubes are worn out, or the bias has drifted low ( A high probability with Factory amp setup and older tubes ).
The easy way is to do some tube swapping with a known good tube or to move the tubes to different positions to see if the problem persists. If you take the PI tube and swap it with V1 for instance, you should get your volume back, but you may have lower distortion from the gain knob, for example. Or if you have a good tube, simply replace it into each position until the problem goes away. As for the power tubes and their bias, you need to know a little more about amps to figure out if they are the issue. I can look at a tube while it's working and tell you if it is biased cold or hot, and if there is an adjustment knob, I can get the bias set by ear and be within safe limits every time, but I can't explain how to do it. In my 20+ years of working with tube amps, I just sort of learned how to hear and see where the tubes want to be biased at.
Sometimes, the FX loop, if it is in the circuit ( like in an Egnator Tweaker or Soldano SLO ), can cause low output if the contacts are bad. If the contacts are dirty enough can still pass signal, but can cause issues. This can usually be figured out by inserting a cable into the in and out of the FX loop to try and clean the contacts up a little. If this solves the issue, then of course a thorough cleaning of the contacts is in order.
I wish you the best of luck. I would start by doing some tube swapping. Keep track of what went where. Work out the problem by only doing one thing at a time, not all at once. You want to know where the problem was, not just fix it willy nilly.
