Try 15's, honestly.
On occasion, mostly nights where my bass player is absent, I'll just throw my 2x15 in line with my T. The Cab the T powers is 4 ohms, so is the 2x15, put it at 2, and let it go. The bottom end fills out, you don't hear much high end clarity, that's what the 12s are for, but it really, really fills out the low-mids, and lows. It's a real thickener.