Yeah, basically any sealed box sub - unless it is severely damped above 30Hz in order to keep the frequency response flat - is going to start to roll off at about 40Hz or higher. If it is just a straight up sealed design with no additional high-pass filter to make the deepest bass roll off even faster (eg. Emotiva's X-Ref subs have an additional HP filter on the bottom end), then you'll get a 12dB/octave roll-off on the bottom end.
To put it another way, if you look at the SVSound SB13-Plus DSP (which is a superb sealed sub), what you'll find is that it starts to roll-off up around 40Hz. The roll-off is not very steep - it's the expected 12dB/octave 2nd order slope. BUT, that means that down at 20Hz, the SB13-Plus DSP is 12dB quieter than it is at 40Hz or above. Subjectively, that sounds somewhere around 4 times less loud - half as loud and then half again. So it's a significant decrease in output at 20Hz vs 40Hz.
Now, with most music, there isn't much below 30Hz. But you talked earlier about wanting some "serious low end" - which I'm guessing might mean some synthetic music - dance music with synthetic bass and heavy bass beats. That stuff can be below 30Hz for sure.
Now, in a small or medium-sized sealed room, you do get some "room gain" - especially if the subwoofer is placed in the corner. That "room gain" can boost the low bass output. A sealed sub with a gradual 2nd order roll off sort of depends on this "room gain" if you want nice, flat, linear output all the way down to 20Hz.
The problem is, it's very hard to predict what is going to happen in your particular room, given that it does have openings to other parts of your home. It's not a sealed, perfect rectangle of a room, so it makes it very hard to predict how the bass is going to react. Personally, I just prefer to have a sub that plays nice and flat down to 20Hz all on its own. If your "room gain" ends up boosting the deepest bass too much, it's a simple matter of bringing down that response with some EQ. But if your sub cannot provide enough deep bass, no amount of EQ can make up for that. So that's just my personal taste, but I prefer a sub that plays flat in a semi-anechoic environment all the way down to 20Hz. I can easily "tame" the room response with EQ. I can't do anything if the bass energy isn't there to begin with!
To be perfectly honest, if you can stomach the idea of paying some return shipping charges, your absolute best solution is going to be to use those 30 day return options that most subwoofer companies offer and listen to some in your home for yourself. That's really the only way to know whether what you buy is going to deliver the performance that you're looking for!
If you go that route, order just one of the subs that you're considering. While you're auditioning, completely forget about practical placement of the sub - as in, don't worry at all about where the subwoofer is actually going to physically go once you find a pair that you're going to keep. Instead, while you're auditioning, just find the absolute best position in your room for that single subwoofer - which might be in some really awkward place out in the middle of your room! The point here is just to find out the best case scenario for the bass coming from that particular model of subwoofer. Once you find one that really delivers, you can use the second subwoofer in the pair along with the first one to get the sort of response in the room that you want while also physically positioning the two subwoofers in places that make sense and aren't physically in the way! The purpose of multiple subwoofers is to create a "phantom" subwoofer - much like a "phantom" center in a stereo image - that is "phantom" located at some spot in between your two actual subwoofers.
Anywho, if you're drawn towards a sealed sub, I'd highly recommend that you try a single SVSound SB13-Plus DSP. Like I said, audition it with no thought to actual placement. Just find the absolute best spot in your room for the audition - regardless of how awkward or ridiculous that position might be. Listen to the single SB13-Plus DSP in that spot and see if it delivers the kind of deep bass extension that you want. If it does not (ie. it mostly sounds great, but just doesn't play quite low enough for you), then you'll know that you want to use ported subs instead. If the SB13-Plus DSP doesn't play loud and low enough for you, no other sealed sub in the same price range is going to do better! That's why I recommend trying the SB13-Plus DSP first
If the SB13-Plus DSP DOES satisfy you, then try placing it in the spots where it might actually make sense for it to live for a long time. You might luck out and find that one is enough
If it isn't, then hopefully the $2400 delivered price for a pair of SB13-Plus DSP will be OK by you, since I really do feel that the SB13-Plus DSP is just excellent - really cannot be beat for the price.
That's how I'd come at this problem, anyway. If you can stomach the idea of possibly paying to return the SB13-Plus DSP, getting one into your home for an audition will tell you everything you need to know.
