Welcome to the forum. Hopefully you've already found the information you were looking for. Your issue could certainly be a cable or tube, but usually once a tube goes, it's done. The volume won't come back and then fade again.The DSL series is notorious for bridge rectifier issues which can cause the volume drop out you described once hot. I replaced the BR on my 100 after having the same issue and it solved it permanently. Due to the high heat inherent in the 401 it should be relocated off the board. Someone beat me to it as it's been replaced, rewired bolted inside the rear of the chassis. Also, if your 401 was built prior to 2004 it there is a Marshall service bulletin that will cool the amp a bit and give it a little better definition by lowering the bias spec.Here's a link to some good info.
Bought used DSL 401