try taking a signal from the effects send or messing with the mix control on the loop. You could have a bad cable if you haven't thought of that already. These things are easy to troubleshoot you just have to be systematic about it. Certain tubes will affect the preamp as a whole: like V1 for the input, the effects loop tube, output tube (if it even uses a tube for that). I suggest getting the manual out and reading up on it for possible solutions. The manual should label what each tube does so you won't necessarily have to waste your time pulling a tube that is not even being activated. Check the easy things first: fuse, cords, settings, footswitch buttons, make sure the tubes are lit up.
You might also want to pull the lid off to see if anything became loose inside. Don't go poking your hands around there but use something non conductive (like a chop stick or a knitting thinga ma bobber). You can use the chopstick to tap around on the PCB to see if the sound jumps back. If it does then you can probably assume a bad solder joint somewhere. An easy way to test things out is to put a cd player to the input and just repeat a song that does not have any drop outs. Take a cord from the effects send and see if you get a signal. If you get a signal there then you know the problem lies after the send and you can troubleshoot further from there. I can type tons of things to check but i hope you can get it fairly quickly.
I think the problem is simple like a bad tube, bad connection, bad cable, etc. Good luck. If all else fails take it to a boogie dealer or trusted amp tech. Some of the local boogie repair guys may be clueless so be careful who you choose to leave your stuff with.
