Sub Question. One higher quality sub or two lesser quality? (Rythmik vs HSU)

Am I missing something here? Please pardon my ignorance but
what about two of the Klipsch  SPL-120?  They've gotten great 
reviews.

My primary speakers are Tekton DI's which have very respectable bass and good efficiency.  Similar to your Cornwalls in that regard.  I also have a pair of the the Hsu subs you are considering.  I feel they would do a great job of filling in that bottom octave you are after.  Love mine and have no itch to upgrade.

It’s been said a million times there is no substitute for quality

just get 2 of the VTF-3 MK5 HP subs from HSU or try to find 2 of the REL S/5 SHO subs used, if you go with the HSU VTF-3 MK5 HP subs you would be better off using the high level inputs from the amp instead of the LFE input.The high level inputs give a more even lower end sound for 2 channel listening while the LFE connection is more boomy and better suited for HT.

Thanks ozzy62, lovin' this thread.
I am considering building using Rythmik sub module and a pair of 12' in a folded dipole design.
My only response so far was that the measurement device may not be the most accurate compared to an accelerometer .. Then I considered a DSP amplifier module moving away from the Rythmik AB to class D and no servo at all. Chasing the dragon....

I'll be reading with interest. Cheers mate.