Sunn Coliseum-300 issues

[ACAB]

I bought this head last summer and it's worked great (cleaned it out months ago too just to unclog the fan ducts) However, t magically stopped working yesterday.

I checked the wall socket, and all is fine.
I checked the cabs, all is fine
I used another head to see if it's the guitar, all is fine and working

I opened it up and turned it on, and the fan is working but I cannot get a sound out of the head unit. I noticed that the power limiter light flicks once then goes completely off after initially switching the amp on.

At first I suspected a fuse, so I replaced the fuse and plugged everything back in, and still the fan works but I can get either no sound or a very faint sound when all pedals are on and volume is maxed (otherwise it's just the fan and the hum from the guitar)

I'm mostly at a loss and considering taking it in Monday to get looked at; but I'm just really puzzled at what the issue actually is.

EDIT:

I also plugged the guitar into the RETURN jack to see if the inputs and outputs were the problem; but stil I got no sound coming from the unit.