I'd say a midrange or mid-high is going to be essential in making this work. I have the much talked about Carvin LS1503 and use it with an amp/FX simulator and it sounds very good. I have to drop the bass EQ almost to nothing and turn the mids and treble up, but it sounds very good. Louder than a 50 watt tube 2x12, for sure, and with whatever tone I want to put through it. It's especially good with AEG's. It really gives the fullrange of tone just like plugging into a PA...'cause it is

Are you using the 3012HO or LF? If you're using the LF, I think you could get by with the Alphalite 6CBMR. The closed back would allow you to get by without building a separate compartment withing the main one. I'd build a tweet into it also with an on/off switch if you play AEG.
Might check out Duke LeJune's Thunderchild cab...it's more or less what you're looking for. Send him an email and see what he thinks/suggests. Cheers!