IIRC, Gordon, the Bose speakers/head unit are specially made for each other and the speakers have the amps built into them. If you have different speakers installed, there is a problem with the volume...there isn't any. Now remember, I haven't had a C4 in over 10 years and my memory isn't what it used to be.....and there have been some advances made in sounds systems since then.
When I had my 94, I had the speakers/amps rebuilt to factory specs and when I re-installed them, they sounded like new. Hopefully, someone with more current knowledge than I, will come along soon to, either, verify what I said, or get you up-to-date information.