Adding an Sub that doesn't go well below speakers?

The backbone of my setup is based on a couple of Canor Monoblocks that feed a pair of Marten Oscar Trio. The trio are rated for 27hz (-3db), but I still feel that the system would benefit of having one or two subwoofers that help both of below 27 while also allowing more breathing space for the speakers.

I was considering a couple of REL T5X because of the little footprint as well as the high line connections (the amps don't have a dedicated low-level sub out). But just before pulling the trigger, I saw that the T5X is rated at 37hz (-6db). 

Would it make sense to add them to the system? Doesn't sound right. I know probably the Martens roll off above 27, but perhaps 37hz sub is too high for my system.

Perhaps a T9X that goes to the same 27hz? Or do I definitely need to splurge and do for an S series, like the S510 that is rated at 20 hz? If I do an S510 I will do just one and add a second once my wallet recovers.

Well, would love to hear from you.