Woofer for a small box WAW / FAST with Peerless TC9FD18-08

I have a pair of Peerless TC9FD18-08 drivers. I've tested them in a 3 L heavily stuffed closed box and they sound fantastic. The best mids/highs I've heard.
And now I want proper speakers made with them, conventional 2.0 design. I want to preserve as much of the frequency range produced by the TC9 drivers as possible while providing assistance in the low-end range.

I need a woofer/subwoofer that will work in a reasonably sized vented(or with a passive radiator) box(around 10-12 L not more than around 15 L) and play up to around 350 Hz.
Also I would like to select something that will go as low as humanly possible with a 5-6.5 inches driver and reasonably sized box.
I plan on listening my future speakers at very moderate levels, few Watts of power, not more than that. I will prefer low-end reach to a huge excursion and power handling for sure.
I will listen to them in the only sweet spot behind my desk.
2nd-order crossover will be active and the speakers will be bi-amped, so no sensitivity match required between woofer and fullrange. HP filter with high Q to assist with the bottom end and to limit the excursion can be easily added.

I'm eagerly waiting for your recommendations.

I've seen a FAST design with TC9FD18-08 and RS225-8, but that 8-incher requires a huge box that won't fit my current living space.
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"huge excursion and power handling" is what I do NOT need.
Lowest practical F3 for a moderate box is the only thing I care. And I don't expect this to be not limited by an excursion for the power over few watts. That's why I emphasize modest listening levels.

Also I don't need any midrange from the woofer/subwoofer. It will be crossed to the fullrange as low as possible.
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Use DIY to your advantage……build a desk with two upfiring 8” woofers built in and suspend your TC9’s above the region of the 8” woofer. Build the desk thick enough for two seperate air chambers to achieve the volume needed. Port it to the sides and call it done.
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I'm looking for a conventional design, so no to an acoustic desk, sorry.

I was wondering what about an 8 inch sub/woofer in a sealed enclosure? If I scale up my requirements a little, then you can fit a lot of 8 inch drivers in an enclousre under 20 L.
Of course, any 8 incher won't play low when sealed, but what about the Linkwitz Transform? How viable could it be? Is operating the driver near Xmax in the bass will produce audible distortions in the broader region?
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are they really as good as that ? I'm planning to build a similar thing for my son to be wall mounted with a 12inch vifa sub and 4 of either the tc9 or faital 3fe25 in a line vertically. I'm torn between the two or the 8inch sb fullrange for half the price . I spose I could try them all and see . what are you using to do you're electronic crossover could I ask ?
I don't think that's the right one . I think that's the version with half the xmax of the one I mentioned . it's this one https://www.walmart.com/ip/Peerless...-08-6-1-2-Paper-Cone-Woofer-Speaker/140580020 they're about £20 in the UK seems good value
An excellent driver for the price: same sound as the 830657 (looks identical) but almost twice the Xmax - but it needs a very large cabinet when ported. Check out "Vagrant Monitor" build on Parts Express Project Gallery for a sealed build with the Vifa TC-7


BTW, here are 24 L boxes with the smallest practical baffle for the 8-inch RS225-8 and the 3.5-inch TC9FD18-08 arranged along my 25-inch computer monitor. Circles show outside diameters.
Would you fit these on your desk?
Here are RS225-8 in a 22 L sealed enclosure incorporating Linkwitz Transform and a subsonic highpass to tame excursion. I usually don't crank up the volume over 93-94 db SPL in a nearfield situation pretty much ever, so +3 db to that as a target was selected for LT correction. Excursion limited with 7 mm at 97 db SPL it results in F3 37.8 Hz, F6 30 Hz. Also 97 db SPL isn't that far from the max power you can put into TC9FD18-08 anyway. But I wonder how much will be eaten by diffraction, and how much gain will come from my small room. But I guess I will see(and measure) only if I pull the trigger on a large enclosure and build this contraption.
On paper it looks pretty good for me, apart from 24 L overall boxes, of course.
Also, real music has lower energy bass content than the rest of the range and usually never reaches 0 dbFS peaks in the bottom end. I've checked a few tracks that I listen to.
In my DAW I've cut with a sharp filter everything above 50 Hz and what is left never reaches -9.5 db and lies in the range of -9.5 to -18(!) db. This means that RS225 will be capable of much more than 97 db SPL total with suggested Linkwitz Transform applied, doesn't it? Give me a sanity check on my assumptions.