So I ended up going with (2) 18 Eminence Tour Grade subs / BSS FDS 310 Crossed over at 80 , Crown Macro-tech 2400 powering the subs. Sounds super tight & focused and I have that concert level kick drum now...I can always ease in the subs or pull out as needed...very happy. 

Some of these Subs like the REL S/812 seemed kinda pricey....an Emminence $250 speaker in a shiny powered cabinet doesn’t seem worth $3200?

Yes it's quite expensive but it delivers.  It's the performance that matters. Some costly hifi may not perform up to expectations and that's money wasted. I have a single S510 and it transformed the listening experience. The old sub that it replaced cost close to $3k MSRP back in 2009 and there's no comparison.

@ryder 

that's awesome and exactly my final conclusion , if it sounds good to the listener , then you have arrived, good for you ! 

from what I heard, for the most part, subs are cheap thrills (cheap subs). Listening fatigue will be off the charts if it’s too deep. So far I heard two types: powerful, in your face subs I don’t like or subtle subs I can’t hear (and sometimes I got it right.) I am waiting for an ELAC to try it with a remote.