I've only been in about one of those, but as I remember it is exactly a PM1.5 but with higher sensitivity RCA inputs. I think there are a few places where you can find the PM1.5 documentation online.
Or you could try contacting the re-born Carver at http://www.carverpro.com/2004/. They do a reasonable job of trying to support older pro product but don't have anything to do with the consumer hi-fi stuff, so if you ask for schematics on the M1.5 they may say no whereas they probably do have them for the PM1.5.
I am no help on the thump-at-turnoff problem, but I can tell you based on many repairs of PM1.5's that there are a few other issues you might want to address if you get inside:
1. Re-touch the solders at ALL output transistor sockets regardless of whether they look bad or not. This can be a bit tricky getting the iron in past the wiring & avoiding solder bridges but is well worth the effort.
2. Re-touch solders at all the hot-glued caps near the right (viewed from the front) end of the channel PCB's.
3. Squirt the bias and power supply trimmers with some DeOxit and work them back and forth (only if you plan to adjust them, otherwise probably best left alone).
Power supply trim is on the front-to-back PCB and should be set to +/- 124VDC measured at the large filter caps on the right side.
Bias is 3.4mVDC measured across the emitters of the two center output transistors on each channel (I find the transistor socket legs a handy place to clip on for that measurement).
Again, this info is only pertinent if my memory is correct on the M1.5 to PM1.5 relationship.
Good luck on the thump quest!