Here's what I think I'd do.
You know that the speakers work. You have another amp on hand.
I'd start by running a cord from the MASTER LINE OUT to your other amp. Use a line in if the other amp has one, but the regular input is okay, too. Just make sure it's turned down, because, if this works, you'll be putting in a pretty hot signal. If the preamp is good, then you'll get sound from the other amp. Might be distorted, but that's from the hot input signal overloading the input. Don't worry about that, at least not at this point.
If that doesn't work, then do the same thing with the individual channel LINE OUTs.
If those all work, then you know the preamp is not the problem. You already know that it's not the speakers, so that leaves the power amp and interconnecting wiring. Never forget that it might just be a cord!
Loudthud suggested plugging your guitar into the POWER AMP LINE IN. I've tried that before and got nothing, even on known good amps. Might be that your guitar doesn't have a strong enough signal to drive the power amp to an audible level. If you know that you're getting a good signal from the MASTER LINE OUT, then running a short cord from that to the POWER AMP LINE IN gives you a good signal into the power amp. Thud suggested that, but I'm not clear from your answer whether or not you tried it. If you have another, known good amp with a line out, go from that line out to the Beta's POWER AMP IN. That's to prove that the problem is in the power amp.
Doing all that should tell you where the problem is. Once you know that, you'll have to decide how to proceed. If you have the technical skills, you might be able to fix it yourself. Otherwise, you'd have to take it to a tech. Only you can decide whether or not that's worth the cost.
