I recently picked up a used DSL40c for a great price, BUT...the first gig I took it to the Power tubes started to rattle and I couldn't use it; No big deal, it's a 2013 amp with original tubes so that was somewhat expected. So I replaced the PTs and re- biased. The next show I used it on, I noticed that when I first turned the amp on, the Clean/crunch channel volume control no longer worked. I could control the volume with the gain control only. The volume would increase slightly when maxed out but when turned down all the way, still very loud depending on the gain setting. After playing an hour or so, it started working normally and the volume control was fine. It also worked normally when I first brought the amp home and tried it. Fast forward to my next gig....same behavior....I tried swapping all of the preamp tubes with no luck, and now the volume no longer functions no matter how long the amp is on. The red channel works great; no issues. Any ideas?