Monitor the Situation
After scamming around on the net for a while, everybody's choice for some sort of affordable flat-response monitor solution seems to be the Event 20/20's. I had to wade through an awful lot of stupid arguments about monitors, but this review by Sound on Sound is pretty encouraging. The only other possible option I've been able to track down is the Mackie HR824's, which seem to blow the 20/20's out of the water, but with an understandable price increase that I probably can't manage at this point in my music and web design career.