This One has Me Stumped

 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. 

@hsounds 

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. 

Similar experience when my source component was overloading my preamp.  I suggest you try every source and see if it's consistent or just a single source.

I suppose the preamp could also be overloading the EQ or your EQ settings are too high relative to the new preamp's signal and overloading the crossover.  Try turning the gain down on everything a bit, including if you have any variable gain on any source so you can be sure you're not overloading the pre. Basically turn it down young man! 

I've taken the xp-12 into the Pass factory. Kent gave me a quick tour of the production area. I explained my observations and measurements to the tech who will be testing the xp-12.

They've had it for about a week. I do hope they have found something wrong.

@jji666 I've measured signal levels throughout my system with an AC voltmeter, osclloscope and RTA. The measured input and output voltages are well within normal range for each component.

I hope that Pass has an answer.

Pass Labs found a gain module that was out of spec. The preamp was manufactured in 2019. Passed replacd the module with the current production module and tested. My XP-12 is now in spec.

I've not set it up in my system yet.

Pass was very responsive and discovered the problem very quickly and was honest and straightforward about what they found and what they would do to fix it.

Excellent service! Very trustworthy.