I've got 2 SVS 12" sealed subwoofers on my short list for audio. It may be that folks who have bought them assumed that they will reach as low as the ported versions.
My opinion on this is that SVS tunes their less expensive sealed subs to behave more like a ported sub. IOW, instead of having a gradual roll-off they have a fairly abrupt "knee" with a fast drop off below it. Here is the FR for the SB1000 (12" - sealed). The "knee" is around 27Hz. This is not "sealed sub" performance, and SVS gives you no way to adjust or tune it!
I am happy to see that the new SVS SB2000 departs from their previous 2nd tier model in that they are no longer aggressively tuning it to try to get 20Hz flat anechoic response. This graph says a lot!
This is a more typical sealed sub response with a gradual roll-off start in the 35-40Hz area! (I do believe SVS adds some DSP, but not nearly so aggressive as the SB1000). If you added the same 7dB/octave room gain starting at 40Hz to the SB1000, you would quickly realize that you have bloated lower bass which will cause your entire system to just sound muddy!
Let's think about this. SVS is aggressively pushing the limits of their least expensive sealed sub to extend anechoically flat down to around 27Hz, while taking their next model with a better driver and a larger box and more or less allowing it to behave naturally.
SVS knows a lot about designing subs, but they are also the most successful ID sub company (I don't know this as absolute fact, but they seem the biggest/most growth). So the only conclusion I can reach is they have made a marketing decision to have the SB1000 impress unsophisticated buyers with amazingly deep bass coming from a comparatively small box at the deliberate expense of sound quality, and that is working for them.
I personally could see using the SB1000 in an all HT system where the sloppy bass would just be more Umph when the dinosaurs stomp (and that is kind of cool and often still not as much Umph as reality - in the case of a near-by explosion). But the last thing I want is more Umph when a low note or bass drum hit happens in music, I want properly balanced Umph!
So if you are looking for a sealed sub for the traditional reason ("fast and tight" bass),my take-away is to be wary of sealed subs which do not have a gradual roll-off starting around 35-40Hz. RoomEQ like Audyssey may or may not be able to correctly interpret/correct this situation (I, personally, have not been satisfied). I am pretty sure miniDSP and REW can do the job, but either way, it is far wiser to start out with a more natural response. If you have a larger room then the room gain would be less, or more for a smaller room - good question for SVS if you are uncertain about your specific room). The Hsu ULS, Rythmik, PSA S1500 and SVS Ultra SB series all offer tuning adjustments to help match your room gain.
If you are wanting a sub to do dual duty for music and HT and decide to get a ported sub, look for the ability to tune the sub with plugs for the ports. These generally do not perform quite so well in the sealed role because the driver is specified to be capable of ported performance (which is a small but real compromise),however, a quality ported sub can do fairly well with music. Here are the FR's Josh Ricci measured for the different modes of the ported SVS PB13 Ultra (there are more tuning options, but this gives you a sense without too much clutter). As you can see the "sealed" mode (pink) does a decent job of looking like a sealed subwoofer:
