My son bought a used Peavey T-Max Dual Channel Bass system head several months ago. It worked great until a few weeks ago. It sounds great right after you fire it up and may work right for 15-20 minutes. After that, it sounds like a thunder storm: loud cracking and rumble but you can still hear what you're playing through the noise. It doesn't matter what you do to the controls; turn em up, turn em down or even if you unplug the bass, it just keeps on rumbling. If I turn it off and back on, it usually comes on still rumbling. Turn it off overnight and it's fine the next day for 15-20 minutes. I learned a lot about electronics 20 years ago so I'm fluent in the lingo but my skills are rusty. I've already tried swapping the 12AX7 tube with no luck and tapping on the output transistors to see if it is a bad solder joint. Also tried re-seating the cable connections. Any ideas?