So I just picked up a used Kemper power head and Matrix Q12 (passive). I'm going from the monitor out to the input of the speaker. Both the input and output levels are fine (very occasionally flashes red but more or less always green) however once I strum hard I get this clipping/overdriven sound. It's sort of harsh/fuzzy and doesn't sound right. This is across all rigs. Reducing the master volume, rig volume and gain doesn't seem to improve things - the overdriven/clipping sound still remains if I strum hard. Also the overall volume level is fairly low. Reducing clean sens doesn't make a huge difference either. And I've got the cab enabled on all rigs.
I've tested the kemper going from the speaker out into a real cab with all settings the same and the overdriven/clipping sound is immediately gone and the volume is also noticeably louder. Sounds great this way. Plugging back into the Matrix cab using monitor out and the clipping sound is back and volume drops. I tried resetting general settings and it hasn't made any difference. I don't think there's an issue with the Matrix speaker because I played through it with the kemper a week earlier and it sounded fine.
I'll try the speaker using something else to make sure it's not faulty. In the meantime are there any other settings I should be looking at?