Seems like you've covered the compatibility issues and the distortion shouldn't be happening. I'm sure they'll figure it out.
The reason I mentioned REW getting confused is that I've had highly audible distortion pop up that was apparently so strange as to be unrecognizable by REW. Flutes sounded like a Kazoo was playing along with them. I thought a driver had a loose spider or something. Turned out to be an odd anomaly in the digital domain.
The 30.8 may have ideal synergy with the XP-12, but the XP-12 should not be causing the amount of distortion the OP is hearing and measuring with any good amp with reasonable input impedance - which he has. Best to take it in and have it checked.