Nuforce STA200 issue. Rumble every couple minutes

In my opinion the STA200  is aa perfect match for a passive preamp. It is highly sensitive (350mV), very high gain unit. I own one. 

It did not work with a Joilda tube preamp I once used. A low gain  active SS pre may work.

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. 

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).