The big VMPS subs are passive and front firing. They are easy to drive and go very deep. I'm driving mine with an Onkyo M-504 amp crossed over at 35hz.
I think you may need help with the room acoustics. Your speakers will rival any sub made! You may want to bi-amp or bi-wire the khorns? Put different feet on them? Try a different amp on them? Remember that low bass will use the majority of your power. However, your other problem here is that, at least in my opinion, mac tube gear sounds better than mac solid state.
Try going to 1 pair of speakers and using the cornwall amps to biamp the khorns. You may get a nice response.