I owned two STA-9:s and was running them in mono operation for the front L-r from my Maratnz SR7010, had no issues at all. No humming, or other issues, dead silent, converted rca to xlr, but when changed to Nord NC500 MK2:s and compared them side by side, I sold the STA-9:s. The sound was from the Nords is so much cleaner. But for the price they are good, and if you like the warmer sound from them they are good for the price they had back then (2016-2017). And NuPrime is not that uncommon what I know, they have many resellers here in Sweden, and rest of EU.
I say, if you can , ask if you can make an home loan and test them in your setup, if you like them, you buy them. They are not bad, though some horror stories on the net, how many units is sold that is still in use, and working without any issue? That you will never hear about.
And if they were that bad the company would have been long gone.