Marshall DSL20CR - No dirt on Classic Gain channel?

I recently picked up a used DSL20 combo - and it sounds great for the most part, however I can't seem to get any grit out of the Classic Gain channel, even with the gain dimed. It does get a bit more compressed and percussive with the volume up as well, but it's still basically clean.

Watching reviews and clips online, it seems I should be able to get a fair bit of grit from the classic channel, but it's just not there. I'm using a Greco Les Paul with humbuckers as well, so i don't think it's an output issue from the guitar.

The previous owner did apparently replace the output tubes with JJ EL34s if that could have something to do with it?