So last night at rehearsal my Marshall DSL 100 started to sound very muffled with less volume and gain. But not all the time. it was muffled for a couple of seconds and then back to normal and then back to muffled etc etc...

I had been playing for maybe two and a half hours so i figured it was overheated and turned if off.
What do you guys think?
Where are you physically positioning it in the room? Make sure it isn't near any heat sources and make sure it isn't too close to walls, and don't cover up the vents.

Also, next time you're doing this, check for any strange smells coming from the amp, and have a look at the tubes to see if they're changing colors or anything else like that.

I've never personally experienced an overheating amp so I can't tell you with certainty that overheating is the issue here, but these are just some basic steps you could take to try and fix/pinpoint the problem.
Ibanez RGA121 | ESP LTD H-1000
Axe-FX Standard
Quote by Callmar
it was re-tubed this year

honestly, that doesn't mean anything.

they could have bad tubes to begin with, there could be something else wrong that is causing them to fail more quickly like a bias issue, could be a bad Output Transformer - it is hard to say.

I'd probably get another Quad of 'Closely Matched' tubes and try again (keep your old ones obviously). Other than that - take it to a tech.

These amps are Cathode biased so pretty much plug n play.