JPJ: Uhm, where's the problem? I mean: So far you've been using the (variable) line output of your external sound-interface -> powered monitors. The (variable) line-out or combined line/headphone-out of onboard-sound hardware doesn't really differ that much from the line-out of your sound-interface, other than being unbalanced only (and possibly being a bit juicier and having a lower output impedance, in case of a combined line/headphone-out). Powered monitors will most usually be prepared to alternatively also accept an unbalanced input signal, so all you'd need would be a matching adaptor cable. Or in other words: I don't see why you'd need your Mac 4100 for that interims solution at all.
Greetings from Munich!
Manfred / lini