Overheating Rectifier and Power Tubes

[crob1565]

Hi, I own a B-52 AT100 and for about the last two months it seems that at least once a practice, after a random period of time, my rectifier, and power tubes 1&3, or 2&4 will start over heating (lately only 1&3) . Right before it happens my low end begins to sink and thats how I know its coming. I've taken pretty good care of the amp. It took a slight fall, if you even want to call it a fall, my hand slipped of the bottom as I was picking it up off the floor once. That couldn't have been more than a foot though. The other incident of neglect was once I plugging in a cable to my effects loop, in the process I unplugged one cable from my cab. I realized pretty quickly and turned it to standby then plugged it back in. That couldn't have been more than 15 seconds though. about 4 months before this happened I replaced the stock Sovtek 6L6 5881's with Sovtek 6L6 wxt+'s and the four preamp 12ax7's with tungsol 12ax7"s. I didn't think there would be any need to bias the amp for that power tube switch. Maybe I was wrong? or maybe my speaker cable incident actually damaged my amp? was wondering if any one could help considering B-52 has not replied to any of the emails I have sent them. Thanks!