I'd go with the MB Quart, but only because I've installed that exact amp for several people in recent months. The Discus amps have been pretty popular for a while now, and I think their reputation is deserved. They really are nice amplifiers for the amount of money they cost.
The Audiopipe could be a decent amplifier too, but I wouldn't feel comfortable recommending it. I've never used any Audiopipe products, and I don't like giving advice about things I'm not familiar with...