Subwoofer to complement ProAc Response D2s


Loving my ProAc Response D2s but I'm considering adding a subwoofer.  Currently have a Cambridge Audio 651A Integrated but watching postings for a Luxman 507 or 505.

L shaped living room where the main portion is 13' by 19'. The left side of the room (the L) extends into the dining room which is 7' by 12' 7". Ceilings are 8'. May be downsizing in the next year to a smaller house which likely means a smaller listening space.  

Had an email exchange with REL and they recommended the T9/i which seems a bit over an overkill, not to mention a bit more than I had budgeted.  Was thinking the new T7/x might be sufficient.  Your thoughts, recommendations?
goethe58