Have to be really careful with the early, pre-2005 versions. I had a couple DSL’s years ago. Bias drift is very common on them. I had to replace the entire PCB. It’s the only correct fix. Marshall sells a loaded PCB for $200 or so.
Just because one of those amps doesn’t have bias drift now, it could get it at any time. Those boards are inferior. The 2005-up have a revised board. I wouldn’t roll the dice on an older one.