After a bit of time away (and a couple vacations), I've finally had a chance to read the replies here. I wouldn't have guess speaker-level output from the Bose HU, but as you describe it, it makes a lot of sense and certainly helps explain what's happening. Thanks for the suggestions and info.

JrFaust, good idea, but no, I crawled into the trunk with a flash light and don't see any adjustments I can make to the amp... if only life were that simple :/

For the Bose HU -> empeg part, should I care about dropping the voltage? For now, I just don't turn the volume up all the way and it sounds fine... am I hurting the empeg, or is this an ok thing to do?

For the empeg -> Bose Amp, I read the description of the second link you provided. It sounds like it's going the other way "convert speaker level output to preamp output". Can I just turn that around to convert the empeg's pre-amp out to be a speaker level input to the amp? Or would I need a different device?