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

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.

I say neither. Get a Rel subwoofer.