No, I haven't heard the audioengines or the various others mentioned in this thread. I've heard mackies, which I also considered buying, but without doing a direct comparison I can't say much there.
Yeah I've got a DAC but it's part of my headphone rig. I could probably try it at some point, I think my computer has optical out that I could use with it, I bet that would help with the noise floor issue. Even a fairly cheap usb dac might do the trick... but again my computer room is so noisy it's not really relevant for this system so I probably won't bother right now, though I may try it just to see how much noise is coming from the speakers themselves vs. the computer. There is a little hiss if I put my ear right up to it even when the speakers are on with nothing's plugged into them, though, so maybe that's just a given with an active speaker. Hell even my main HT speakers (active crossover but not active speakers per se) have a tiny hiss when there's no signal.