One sub can be used but definitely needs variable phase if you are limited with placement options. SVS is a great choice because of the adjustments available. It has a simple download which means you can tune it in from the comfort of your listening position via your cellphone.
In addition to phase you can also adjust volume, low pass filter, polarity, parametric EQ, room gain and presets.
You can also consider Rythmik or GR Research. there is no remote adjustment available so that needs to be done on the plate amp itself. These are servo controlled and perform superbly.
I do not understand those saying a single sub will not help. They can and do but if you just plonk a sub down where convenient then do not expect great results. You state room for one sub only and if that's the case then that's is that, but if you can squeeze another small sub in behind a couch or double as a side table or as stand for pot plant then the overall sound will improve far beyond expectations.
Even a small 8" sub will work. I use them as tuning devices and have used as small as 6.5" driver as a sub. What multi-subs do is smooth out the room response so even the little 6.5" unit that goes down to only 50Hz will help all nodes upwards from there. A 6'5 " driver in an 8" cube should not be too difficult to shoehorn in 'somewhere'