The acoustics of my basement suck. I have tried the workarounds I can wrangle, but it's time to get drastic.

Most accurately, the bass response blows. My basement is large, and runs the length of the house. The largest room in the basement is L shaped, spanning the length of the entire structure. If it makes sense, the bottom leg of the L is the theater area, and it is 15x17. I think the area of the room in total is about 1150 sqft or so, so its overall a large room.

Maybe this won't get formatted to death...

This is actually quite adequate space for my setup. I have a 120" projector screen, 7.2.4 setup from a denon 3600 and klipsch RP series speakers with monoprice Ambers in the ceiling, and everything points just right at a three seater couch.

But the room is big, square, not much in the way of room treatments beyond a couple of rugs, and the drop ceiling comes in at 7'2" (which is a bit low for my ceiling mounted down firing height speakers). The floor is vinyl plank with a built in underlayment on top of concrete slab. As a result, my subwoofers don't give me a lot of bang for the buck. It doesnt help that they're really mediocre (two klipsch r112sws) but every now and again you have that moment of clarity where a tone hits you just right and you realize what you're missing. The layout of the room also prevents me from positioning my subs in a way that works the best (as I would have to sit directly under the projector and to the side of both the rear surrounds and far from any sort of position that would be amenable to the side surrounds either... and the atmos ceiling speaker effect would also be lost).

I do have four tactile transducers in my couch, which does help scratch that itch a bit. But every now and again they do something weird that breaks immersion... I have every intention of keeping them- they rock- but I feel like I run them with a higher crossover than is warranted just to cheat some bass "feel" out of my system, and while 90% of the time they do great, sometimes they're sort of wonky. I watched the streamed version of LOTR on HBOmax and I had to go cut back the crossover substantially and the gain as well as they were just going nuts even in mundane scenes.

So I have a few ideas, and I wanted to get input.

My first idea is a "floor riser". You don't get substantial tactile response off a solid slab floor, compared to in a theater on another floor of a house.  What about raising the floor?

I can remove my drop ceiling and sheetrock the theater area. Why my drop ceiling is so low I don't know, but after shimming the joists for a couple of water pipes, i can have an 8'4" ceiling or thereabouts. If i build a riser in that room, it would likely be a 2x6 frame spanning the area, with OSB only subfloor, and berber carpet or something on top. It would be about a 6" riser as a result. The end result would still be the sensation of a room with a relatively higher ceiling, while also hopefully giving me some of the shake I want.

I could then plant a pair of 18" Daytons or similar behind the listening position. As it is, I'm only using one sub out on my receiver although I have two subs (and four transducers hooked to another sub amplifier) daisychained. I could potentially swing a 21" sub too but I'm not sure I'd enjoy that.

Aside from that, I think it would also look pretty cool. You can enter the room from the rear of the theater, walk up a short ramp to the raised platform, and then step down to reach the rest of the basement. I think the overall idea would be fairly good looking, and I sort of like the idea of visually distinct subdivisions in that very large open basement room. It also has the side benefit of elevating my speakers from any water that may find it's way to the basement. I've only ever had substantial water entry once- after a direct hit by a hurricane- but what if a pipe bursts or something? Elevating the valuables seems like a good idea.

Has anyone else tried building a raised platform for acoustic and tactile reasons rather than just viewing height?

I also intend to add a layer of sheetrock to the walls (currently clad in very ugly plywood panels with a wood panel veneer- think 70s chic but built in the late 90s), quite a few bass traps and absorbers, and possibly stuffing the riser with mineral wool and possibly the ceiling as well when sheet rocking it.

How much effort have you put into getting a flat response from 100hz and down?  If you periodically get the perfect bass, then you at least have a small part of the frequency response there.  You just need to focus on the rest.  Have you measured the frequency response of your room with REW?  At the seating position, my subs are flat to 17hz + or - 3db.  In the back corner where I put my elliptical, I have about a 6db boost at 40hz which is great since I usually watch music videos and concerts while I work out.  I can stop off the machine and walk to the front of it to unplug the power cord and there is no bass.  It's like someone just turns off the subs.  So you may have your subs in the wrong location based on your seating position.

My house has about a 4,000 sf. slab and I can get a tactile response on the opposite corner from my theater room.  You already called your subs "mediocre" so upgrade.  You mentioned Dayton 18s and those will definitely work.  Are you leaning towards any particular design?  You also mentioned that you probably wouldn't like a 21?  Any particular reason other than box size?

Building subs is my hobby.  My current ones are basically this design with a Behringer amp pushing them.  http://www.danmarx.org/audioinnovation/tc3000.html  The downside is that these subs have a 14db peak at 55hz due to the voice coil inductance.  They sound like a crappy car audio system with music and no bass at all in movies.  I measured my in-room response and added a parametric EQ (Behringer Feedback Destroyer) to level things out.  At my listening position, I can hear full details of someone playing an upright acoustic bass and also feel the house shake during action movies.  It's all in the EQ.

I have tried EQ with only middling improvements for bass. The shape of the room unfortunately limits placement somewhat, so I am hoping with some mild remodeling I can address that.

My subs don't go nearly as low as yours. The klipsch subs simply aren't good. I like their speakers a lot, their subs are just not great. On the plus side I paid next to nothing for them; on the minus side both of them suffered amp failure and I swapped them out for cheap amps from partsexpress that turned out to be upgrades (but they still aren't great). I use audyssey multeq xt32 for room correction. I have not tried using REW and a mini dsp or similar due to that. I am pretty happy with my non-subwoofer sounds and the layout of my speakers follows the atmos form guide very closely, other than my ceilings being slightly lower than the ideal.

If I do a dayton build I'd probably go with ported full martys; building that design would be easy although I admit paying current prices for MDF would piss me off.

I could possibly fit 21s to the room at the rear, but I'd like to get that layer of sheetrock added to the walls and some room treatment done first. I am hoping to give that a shot in the next month or two but I'll admit the idea of paying $25 for a 2x6x16 just sort of pisses me off even more than the MDF for a subwoofer box.

The other thing about adding a 21 is that I would probably do 2 at a time, having long since been sold on the virtues of multiple subs. I'm a little concerned I'd need to add a new circuit at that point. That wouldn't be the end of the world but going from the 18s to 21s and then also adding a new circuit for the additional amplifier(s) would add a lot of cost.

Crown XLS amps are very efficient.  I use the older 1000 models for my mains.

My extra 18 has t/s specs that make it perfect for a full Marty.  I also have an Exodus Audio Maelstrom 21 that should work in my new house.

Since you already know about Parts Express get two of the 21" Kraken subwoofers and drive them with separate monoblock amps.

https://www.parts-express.com/Dayton-Audio-HTS545HE-4-21-Kraken-High-Excursion-Subwoofer-with-5-Voice-Coil-4-Ohm-295-054?gclid=CjwKCAjwvMqDBhB8EiwA2iSmPBW5KYbvy-teA2ci7Oi6WQFeQPOMFIhciY2BGezQ-A55-bb96BnABhoCx3AQAvD_BwE

Have your tried doing a bass crawl?

https://www.audioholics.com/home-theater-connection/crawling-for-bass-subwoofer-placement

That may help you some before doing treatments or going the route of a raised floor.  With a raised floor you may get some unwanted frequency boosts, but EQ should help if you go those. Personally, I would do treatments like carpeted floor and acoustic tile treatments on the wall before a raised floor. From the dimensions you gave, the odd length and width are a benefit. If anything, your could close off the area with a wall to seal your theater versus dealing with the acoustics of the other part of the "L".