Um, I just looked at a bunch of suggested Midbasses. Manufacturers encluded Vifa, Peerless, Seas, Resonant Engineering, and Adire. The Adire Extremis 6.8 and the Resonant Engineering XXX 6 inch drivers, on the whole, displaced more than all the other drivers I looked at, in some cases almost double a compariable driver. The Resonant Engineering and the Adire drivers are pretty much identical, save for a slightly different magnet and a different impedance rating. The Thiele/Small parameters are slightly different, but you probably couldn't tell the difference. When it comes to bass, there is no replacement for displacement.
I say, go buy a pari of XXX mids, you won't be disapointed.