i find it strange that breaking the direct chain between your audio interface and monitors with a low-end soundcraft actually makes the sound cleaner. If anything, i would of thought it would add more hum and feedback. I guess these kind of things are very personal though. Having said that, i really dont believe that a Mackie 1604 will give it that extra punch that you want. I have heard wildly mixed reviews about the 1604; some people seem to swear by them, others claim they use the mixer as an ashtray.

You can try looking into an Allen/Heath Mixwizard series. Apparently they are perfect for the home studio environment as it doesnt take up a whole lot of space, and its quite good for both home and live use.