I'm struggling with this thread.
@Hostility wants stronger sub bass output. He has two 12" subs, and is considering replacing them with a single 18" sub with a potentially lower acoustic F3 (as opposed to artificially DSP-extended) in an appropriately designed enclosure, with the option of building a second one down the road. Doesn't sound like a bad idea to me.
Then there was some vague bs about precise calculations and complicated math, implying that satisfactory sub bass requires reading Floyd Toole's book and selecting a specific sub mated perfectly to the exact dimensions of the room, that anything beyond the least common denominator is a detriment to the sound; and that shaking the foundations of the building is unacceptable for enjoyable sound quality (even though that enjoyment is purely subjective). Then a suggestion that the Dayton 18 HO would offer no substantial improvement, that the Ultimax would be a better choice, and some stuff about ARC, and some other stuff about a 15A circuit being incapable of powering anything stronger than what OP currently uses, and then I just couldn't take it any more. I sincerely apologize for stepping on toes, but I don't think any of this advice is practical for the OP.
A single 18 (Sd of 180in²) is physically capable of moving more air than his two twelves (combined Sd around 160in²) even without upgrading the AC circuit amperage. This is good, since OP described a renting and landlord situation. I seriously doubt rewiring the breaker box is an option here. Anyway, as a rough illustration, here's a graph of Josh Ricci's CEA-2010 measurements for a sealed Dayton 18 HO versus the SVS PB12-NSD.
That's sealed. A vented Dayton 18 HO will have even greater output < 40Hz. Seems to me that this will achieve the desired result.
The best advice in this thread thus far has been to get a miniDSP. With that you can shape your curve however you like, and it'll also let you handle the low pass filter as well as an infrasonic filter if you need to limit excursion below tuning. See the link in my sig for my CSS SDX12, and you can see an example of how profoundly you can modify your curve using a miniDSP.
Indeed, if you add a miniDSP to your existing setup and play with your house curve a bit, you might discover that your PB12-NSDs are sufficient to achieve the sound you want, and don't need replaced. Maybe you like a 3dB per octave slope to pressurize your ear canals when approaching infrasonic bass. Maybe you prefer a shelf at 80Hz to feel a kick in your chest. Of course if the peaks produced by your existing subs are clipping and you truly do need more headroom, you can reserve the right to build your 18s later. The miniDSP will still be of benefit regardless of whether or not you DIY a Dayton 18 HO.
The only drawback to moving from two 12s to a single 18 is that you narrow your sweet spot. Multiple subs are preferable to a single for better consistency across a wider listening area. So, yeah, if you do DIY, then don't stop at a single sub.
I'm hoping
@Haoleb will return and restore the build images on
www.haoleb.com for his
Dayton 18 HO build thread. If not, then you can still see the images
on archive.org.