Ok, I think I have some good news...
I just tested my stereo rear speakers. This is a 2012 SRT8 with the factory Boston setup. It does not have the upgraded HK setup.
The rear speakers do NOT attenuate after level 22, in fact they got louder and sounded good up into the 30s. I think I got as high as 36 but some distortion was present. Anyways, it sounded loud and clear with plenty of bass and mids even highs. So it is a FULL RANGE speaker, and it did NOT attenuate. If I got a phone call of course it switches to another audio u-connect channel and it plays the call at a separately-configurable volume just FYI.
So I think I will be plenty happy using the rears as speaker-level inputs into my 1000wrms Alpine amp and TypeX sub setup. Something to work on this weekend!!