It sounds like you've probably done a thorough job of going through the setup menus. But just to be sure, have you optimized all of the settings that may be relevant? That would include "listening position," "smooth," "surround," "live," "sound conditioner," "loudness," and "SDV" (speed dependent volume).
Based on reading their descriptions in the manual I would expect that you would want to set "listening position" to "front," and to disable most of the other functions. "Smooth" and "live" being possible exceptions.
I have the Bose system in my Cayman S, btw. In that vehicle and for my purposes (pretty much just listening to satellite radio) it is quite satisfactory.
Regards,
-- Al