I've been into AV equipment since at least my teen years in the 90s, setting up a $400 Aiwa 5.1 system in my bedroom! Those were the days, a 27" CRT, VCR, and plastic cabinet paper driver speakers hanging on screws in the drywall in my 10x10 bedroom, lol.
I'm I guess what you'd call a 'prosumer' in terms of theater equipment, I've got a new (old) house and planning a 'real' home theater in the future, but set up temporarily in one of the half finished rooms! I'd say on the ladder I'm somewhere between asking people for help at Best Buy and just tickling the underside of many of you who are comfortable with manual room EQ and all the minutae of truly Professional setups.
A bunch of years ago in an apartment I moved up from my Aiwa 8" ported sub (I hear some of you chortling) to a Bag End Infrasub 18, and figured I'd be all set for life! The bass and the difference was truly amazing, but I never got to really open the throttle in an apartment complex. Once we moved into a house and I could put the knob wherever I liked, I've found the point at which the Infrasub would reach thermal overload and shut off a little underwhelming. So, naturally, i bought a second one. NOW I've got to be set for life, for sure. 2 18" subs? Personally I don't know and haven't met anybody (even in 2 years as a service tech for a high end home automation company) with that much air moving potential. No doubt that sounds quaint on this forum, but still.
Suffice it to say, the 2 subs are great but still clip occasionally just below the point where I'd like to set the volume. Running powered DC fans over the integrated amps has only slightly improved the issue, which has led me to the conclusion that there's still room for improvement, and ultimately pointed me to this design. But my personal drama isn't quite finished yet! Initially I was going to just build one THT as a supplement, to be the 'hard hitter' in the room, and fill out the response with my existing 18's. Then, somewhere online on an especially cold, particularly dark night, I read the words, "the only rule is to NEVER mix horns and direct drivers"

I sat there for a couple minutes, rereading, trying to comprehend and come to terms with I was either not building this THT, or, I was selling my 'prized' Infrasubs. I sought confirmation elsewhere online - and even found Bill making the point clear in one of his posts. A younger, prouder version of me would probably try it anyway, but reading through hundreds of posts on the forums here it's pretty clear that folks know what they're talking about, and I see no reason to risk the drivers and audible wackiness for the sake of keeping my poor 18's.
So that's me, and where I'm at - starting the build of these THT's, hoping there's one or two people in the local area looking for some big DD's for christmas. (direct drivers) I'm sure I'll be posting my noob questions on these forums as they come up! Happy to be part of the community.