10-06-10: Bob_reynoldsRight you are. I meant to recommend a sealed ML, and when I saw a $500 ML sub I got too excited. I really meant something more like ML's Dynamo 700, which is 300 watts and a sealed enclosure. While it's a little over budget, it's within reach at $700. It's also roughly a one-foot cube, which should help the WAF consideration.
Johnnyb53, with all due respect, the OP stated he wanted to "make a large step up sonically." A 10" driver in a relatively small 30 pound box is not going to make clean bass at anything much above a whisper in a small room. Any sub can be boomy with the right (wrong) setup. The ML sub you referenced is ported and as such we can expect its group delay to be higher and therefore, sound "slower," than a sealed design.
I was figuring that the volume needs would be modest since the MG12 is one of Magneplanar's smaller models.