Amp displays creeping volume gain


While my NuForce class D monoblocks are out for repair, I’m auditioning the Optima Nuforce Class AB STA200 amplifier.
After break-in, this $499 unit displayed amazing quality for its price to the extent that I decided to replace my much more expensive monoblocks with it.  However, the day after it’s incredible performance, the gain kept increasing to the point that I had to set my pre-amp down to eight o’clock in order to avoid ear splitting volume. The quality of the sound was severely compromised with this setting. I read a review in Absolute Sound which mentioned the unusually high gain of this amp.
Has anyone else experienced this creeping volume gain phenomenon with an amplifier?
I don’t know if it’s faulty or part of the break-in period.
But at this point I’m certainly not replacing my monoblocks with it.
128x128rvpiano