Pair of Rel S510’s or Carbon Special’s?

I’ve been considering adding subs to my system and am considering whether to go with S510’s or Carbon Specials…. Any thoughts? Here is my situation:

-Very large open room (open concept) with high vaulted ceilings

-B&W 802 D2’s

-Gryphon Diablo 300 amp

-Looking for MILD and subtle bass support for my speakers in my big room. For reference I tried the new, larger B&W 801 D4 speakers with my amp and thought there was way too MUCH bass…

-Despite the large room, the layout makes large subs out of the question. Carbon Specials would work. But size-wise would prefer the slightly smaller S510’s

-Don’t care about home theatre

-I do care about speed and bass definition and quality.  The more bass nuance and detail the better.

-I don’t care that much about getting down to the very lowest frequencies. Just need a bit lower than what my 802’s do

-Subs will be positioned about 6” on the outside of each main speaker. This is the ONLY place they could go…

-I’ve invested heavily in quality cabling (Nordost Valhalla 2 speaker cables, AudioQuest Dragon power cord, etc) and have a USB reclocker and network isolation switch. Just pointing out as all these elements have added precision, ambience, clarity and space to the sound of my system.

Anyone with experience in both the S510 and the Carbon Special have any thoughts on which way to go?