It is a bit of a balancing act. Having a ton of low frequency energy available is great for HT, but I, personally have experienced subs that once room gain was added were ridiculously over-blown and Audyssey XT32 alone was unable to tame them (it is limited in how many dB it will reduce).
So if you get a ported sub which will have an essentially flat response to 20Hz territory, once you put it in a room, it is pretty sure to become bass heavy.
Generally, for HT, I run my subs 6-8dB hotter than I would for music, because I am sensitive to boominess in music, but not so much for HT.
I am no authority on this, but here is what Josh Ricci (who is arguably the highest authority on subs) said about the sealed PSA XS15se sub:
The basic frequency response shape with the low pass filter (crossover) bypassed shows a response that is cleanly extended up to 200Hz and beyond, with a gently sloping low end that corners at 30Hz and appears to enter a sealed systems natural 12dB/octave roll off below that point. It should be a good match with the boost often seen in the low bass once placed in room.
Had you not mentioned your concern about boominess, I would have passed by this thread since you are HT oriented.
Boominess is often a factor of too much bass exciting resonance(s) in your room. Resonance is a non-linear phenomenon - where a small increase in at the sub becomes a large increase of SPL in the room. What this means is adjustments can get touchy (kind of like feedback in a PA system).
Unfortunately, this is dependent on your exact room and the location of the subs, and there is no easy formula for evaluating how much room gain you would get at which frequencies. However, we do know the smaller the room, the more gain you will get.
However, Josh Ricci's comment above establishes that it is worth considering the difference between a flat in-room response as opposed to a flat anechoic response.
Edit: I just looked at the data-bass charts for the SB12-NSD and it appears that SVS has very aggressively EQ'ed this unit to attempt a flat anechoic response (which is probably a smart marketing decision) so that undermines the reasons I would suggest a sealed sub. Accordingly, I am not so supportive of the sealed sub. Whichever you get, just plan on getting a miniDSP (but give Audyssey a shot first, of course)
Aggressive EQ results in ringing and an increase in group delay and sloppy impulse response.
Here is the SB12-NSD which uses aggressive EQ:
Here is the PSA XS15se which applies a modest bit of EQ