Ah, but does it really scan the I-Bus, or is it simply designed to automatically supply the correct output for either high or low impedence (i.e. amp or speakers)?

This is relevent as in my situation (no amp) the radio powers the speakers directly, but if it is connected to an amp instead, does the level drop to what could be called line level (ok, with a 5V bias) or will it still be trying to drive speakers, which will surely fry an amp with line level only inputs?