In any case the reason why I'm going for "Frankenstein" subs is because there is a reliable speaker fabricator in my city (well, on the opposite end of the metro actually - we don't have the same mayor) that can help me out with this, especially with crossover designs and finish (not likely as I plan on using paint, for reasons that will be discussed below). I want to explore omnidirectional speakers given the room shape which is a a very wide rectangle, meaning there will be people too far from the opposite side mains; this is in the house I'm in right now but even a larger room closer to a square if I build my own house later will still benefit from properly-designed omnis. That same shape of the room can also be a problem for the sub. Unless I can put the sub in the center of the room. Like I said a few posts ago, even though a sub is generally omnipolar, it doesn't mean it is impossible to localize.
The other reason that I forgot to mention as to why I am designing the speakers as such is because if I do get to build my own house, I want the living room to be the HT room. Again, I am not after absolutely perfect acoustics, but I am still minimizing very problematic ones instead of just throwing an HTiB in there, but the main priority for me is that I see HT as a very social thing where people can sit down, watch movies or play video games, and have easier access to the kitchen (unlike my 2ch rig which will be hidden away in a room inside another room for sound isolation, where I can listen loud or be tortured in a home invasion and no one will hear me). Putting the HT in a room, more so a room on the upper floor as in the library that I might convert (thanks to digital books) in this house, means that we are too far from the kitchen, and I'll have to tell everyone to get rid of their shoes before going up the stairs since we don't track outdoor muck from under our footwear around the upper areas; not even our pets can be there unless they're fresh out of the shower (and all litterboxes are downstairs). Speaking of pets, there's the practical reason for the omni speakers: it's a lot easier for them to scratch the drivers if they're on the front. No speaker grill will stop a cat with claws climbing up them, whereas a grill to cover the space between the diffuser and the driver will have more space between them and the surrounds. Finish on the speakers will necessarily need to be tough while maintaining some degree of elegance inside a house - like coating the wood enclosure with fiberglass to protect from moisture (you never know when they'll cough up a hairball on it, or piss on them if they're sick) then coated with auto paint or something.
Oh and one more thing with "Frankenstein" subs: the enclosure design. Even the subs great for music are designed with the need to make Transformers and LOTR exciting for buyers, but from my (admittedly not as wide) experience with home subwoofers, I would need to spend a lot of cash just to get one that can keep up with the mains on Dream Theater: Live at the Budokan and Kamelot: One Cold Winter's Night, like JL Gothams. If I "Frankenstein" my subwoofers, I can use a relatively high-Q sub that does well in a sealed box if necessary for my goals (although a small ported one will do, but have the slot port exhaust on the same side as the woofer baffle), design it to reinforce the lower notes on really fast double-pedal bass drum action, while the surface area of the sub for the most part as well as turning it up when necessary (movies like LOTR, Star Wars, Transformers, etc) will still have a heck of a lot of improvement over not having any subwoofers (even with quality towers). Essentially, instead of, say, JL Gothams, I can just get a used pair of 8W7 or 8W3V3 (just as an example; I'll still use home audio sub drivers) and get them on enclosures with the aforementioned design parameters.