IME imaging does not suffer at all with a down firing sub woofer. Adding a couple of subs is a great way to go. To the OP, in your room size, i would probably start with one T5x and then see if you can add another.
REL T7x or T5x for Small Room
I am looking to add subwoofers to my system. My listening room is approximately 10' X 10' and I have a pair of Harbeth P3ESR XD's. The room has wall to wall carpeting with padding.
The T7 is really overkill for the room, but has a front firing driver and passive below. The T5 is a better fit for the room but only has a bottom firing driver. I am concerned that I may lose some potential imaging with a bottom firing sub.
Are my concerns valid?