Doug, I see what you're saying, but in that case any amp would be able to handle speaker level inputs and that cannot be true otherwise there'd be no need for high level (speaker) -> line out converters.

Let's forget the 5v bias of this BMW system, it is certainly not true that a line level signal can drive speakers and although far from being an electronics expert, it is my experience that indeed you should not send the speaker level ouput of one amp straight into the line level input of another.

I'm not being trying to be argumentative about this, I just want to be able to understand, partly as I want to feed the output of my radio module into my Nakamichi amps (line level input only) and partly just because I always need to understand with what I am dealing as I am then better able to figure out any variations I may need and/or fix any problem that arise subsequently.

Some facts would help me here. A traditional line level signal would be about 2v, with some system up to about 4v (IIRC Sony started doing this with their ICE), but what would typically be the equivalent at the speaker?