I have a 12" type r with a mrp-m850. it sounds decent in the car except for high notes (doesnt even sound like it hits at all) but it will sometimes make a pop/burp noise... It happens more and more when i turn the volume up and if i listen to it from the trunk it sounds really bad...
maybe bottoming out, i had a 10" type-r on that exact amp set to 750W RMS on the DMM and it sounded good, got decently loud for one 10" sub. Get a DMM and set your gains maybe there too high and clipping the signal