Hey Rob,
I have always used at least one 15" speaker in my rig for exactly the reason you stated. With the Barefaced cabinets I don't miss it at all if anything you have to learn to control the ultra-lows when using these cabinets, the 12" cabs are basically flat but are very responsive to EQ. The 12" drivers were designed by Barefaced, manufactured by Eminence and exclusive to the Barefaced line. My 2x12 SuperTwin (no horn) has response down to 37 Hz, my Big Baby II with one 12" driver, no horn has response down to 30 Hz. The Big Twin II has 2x12s and a horn and shares the same frequency response as the Big Baby II. All their cabinets are super efficient, loud, and capable of handling huge amounts of power. Light and easy to move, too. I usually don't care for horns but the ones in the Barefaced cabs sound really smooth and you don't hear the crossover. The crossover is continuously variable from 2 kHZ to 10 kHz so if you want to send the majority of the signal to the 12" driver you can do so. They are expensive but I find them to be worth every penny.