I've owned one (and then two) SVS SB12+ which were a step up in quality from the NSD. They were nice subs but I ultimately required more headroom when I was building a system in my main system (large, open floor plan with no way to pressurize the room).
I then traded for a pair of HSU ULS-15s. These offered plenty of bass but they tended to be peaky and ultimately required the use of a sub EQ (SVS AS-EQ1). Loud, but better suited for home theater than 2-channel audio.
Both HSUs and EQ were then sold off for a single REL 212SE. The REL was the easiest to integrate and offers a seamless comparability with my mains. It's smooth when it should be, and bombastic when called for. I realize this is up the line from the T/5x, but I feel that the REL would provide some of the best quality sound if not all out quantity.
As suggested by 2psyop, the Rythmik line also has a loyal following and would be worthy of researching.