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

If you have large full range speakers that go down to the mid 30s, if you add subs, you need larger ones that deliver output below that range in that there is little to gain to start with otherwise and you don’t want to merely add to the frequencies already there.

Get 2 Rel’s

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