And I think we have a winner! http://www.mackie.com/products/hm54/
I think two of these would do it, and they're $99 a piece =)
I would like to add something for the RME, MOTU, and Presonus R&D folks here. I think there is a serious need for a device that does this sort of thing. Ideally I envision a device that would have 6 or 8 stereo inputs x 6 or 8 headphone outs with amp + two main in/outs that your powered monitors could be connected to and controled via a volume knob. Kind of a universal cue mixer / monitor contoller, rack mountable of course. While you're at it, add a talkback and make it less than $500. =)
The Hear system is nice but not really designed for us home studio folks.