I'm awaiting my warranty replacement Monoprice Monolith amp. I think I mentioned a while ago that the unit was making a humming sound when powered up.
The Monoprice people have been great to work with. I sent them an email detailing the troubleshooting I had done, they had me try a couple of other things (which I had tried, but forgot to detail in my email), and the sent the RMA info. They paid the postage both ways, and sent out a replacement unit the day the received my defective one. They even let me keep using it until new units were back in stock, even though it was beyond their normal 30-day return window. I'd have no qualms or reservations dealing with them for any future major purchases.
And I still have that fourth sub to set up.