Just came across this exact issue last month. It was the ribbon connecting the master fader to our computer. We just recently switched over the THD-Labs' Tangerine Audio Interface and had to modify a ribbon to get it working again.
Every symptom you had, we had. So I'd say it's for sure the computer. If it's not connected the master fader won't control level. It's like that on our 8000G+ (ultimation) at least.
