So I got a svt classic on craigslist yesterday. Tested it out to the max. Everything seemed fine. Got the head and a 410 ampeg he cab all for 850. I traded the cab into guitar center for 210 bucks and bought the 410hlf. Today I plugged in and the volume on the head is going up and down. Playing at a louder level after a few minutes it cuts by about 50% volume. Also, when cranked up I noticed my brand new ampeg 410hlf was distorting. Sounded like the speakers were crackling. So I hooked up a different head and cranked it up. No issues with a diff head. No crackling and the cab sounded tight. My question is...what cause the volume on the svt cl to go up amd down and if the head is in need of repair, can it cause that farty crackling noise in the cab?