The CDM 7NT is rated down to 40Hz (-3dB), which is certainly pretty decent, but there is material in movies that goes all the way down to (and often below) 20Hz. With no sub, that really deep bass is redirected to your fronts - where it is promptly lost in the response roll-off of the speakers.

I ran my system without a sub for a couple years or so (Paradigm Studio 60v2 mains that have a -3dB point pretty similar to your CDM 7NT's). When I finally added a subwoofer to the mix, the system was still in the living room of our old house (11'x19'x8'), and it was definitely a worthwhile upgrade. Adding a subwoofer will only be noticeable on certain types of music, but it will be a definite improvement with movies. Even if you only go with something small, like the LFM-2, it'll be a nice contribution to the system.