Eminence Omega Pro 18's give you 25 Hz extension but you have to build exactly the right box, with the tuning actually at the resonant frequency to control the driver's tendency to overexcursion at the highest drive levels. The thing is, the Lab12s make such lousy direct-firing pro subs because they are, by themselves, woefully inefficient. They follow a model similar to today's hi-fi subwoofers where they use a really heavy cone and a really heavy voice coil and a long-throw suspension that has a lot of damping in it. They work well in the horn because the horn is hyper-optimized to get lots and lots of output from them, because they are strong drivers. It takes a large amount of pressure to counteract the motor's action. But efficiency is the key for a direct-radiating box. With both those drivers, you're probably only getting a net 91dB 1w/1m sensitivity. With the Kilomax 18, you would be getting 98dB 1w/1m sensitivity with higher power handling as well. And for a pair of 12's, that is not a reasonably sized package. Boxes look well-made, though.