Nuforce STA200 issue. Rumble every couple minutes

it can be a difficult amp...but great sound and incredible value in right system...

Yes, I agree it has a great sound. It is that the sensitivity and gain make it not a universal pairing. Likely why it became so highly discounted. Maybe a good fit for a vinyl system with lower output cartridge and phonostage.

I am using mine with a 

’volume control on a box’ preamp driven by a 2V output DAC. 

The STA200 is an incredible amp, given the " proper system " it is paired with, and of course, the $499 selling price. People are giving them away it seems. 

Yes, people are giving them away because the amp doesn't serve their system. I have been thinking of selling mine. 

Another possibility would be to pair it with a DAC with an analog volume control. 

Another issue is that it isn't meant for speakers that represent a difficult load to a speaker, providing the same power at 8ohms or 4 ohms( 80wpc). 

do you guys know of a amp with similar sound characteristics to the STA200 with much lower gain?