Brent,Just some quick back of the envelope scratching, but here's another $500ish DIY option...well $514 according to my shopping cart at PE. Free shipping currently at PE.
Get the 3 ft^3 finished subwoofer cabinet, the DVC15 passive radiator, RS315-HO, and BASH 300. Add 700 grams to the PR for a 21hz tuning based on a 2.5 ft^3 net box after driver/PR are added. If I've over estimated the displacement of the driver/PR, then more net volume will just lower the tuning for an overal improvement. This will most likely require a separate cabinet for the amp since the internal brace in the PE cabinet looks to only allow room for the driver/PR.
WinISD predicts 105.5dB @ 20hz, 1m groundplane...or 99.5dB if you subract 6dB to approximate Ed/Illka's 2m measurements. Relatively flat at that level all the way up to 108.8dB @ 100hz @ 1m. The 1m output is still good for 95dB @ 15hz. Overall, output at 20hz is comparable to the Rythmik and a little less at say 50hz, but the DS12 kit would be around $565 if used with PE's 2 ft^3 sealed cabinet.
I plugged in your numbers and am not seeing anywhere near the same response. I am using the latest WinISD and the parameters are taken directly from PartsExpress' CLIO data:
Vas 43.38 l
Dd 250.0 mm
For the DVC 15 passive radiator (part 295-194) I have:
Sd 805 cm^2
Really odd thing here is adding mass to the driver is just going to make Fs lower. Also, how do you add 700g (greater then 1lb) do a passive radiator?
I get 97.3db @ 20Hz and frequency response is dropping like a rock at 20hz (second "knee" in the curve). This is the "Maximum SPL" curve.
By the way it looks like Rythmik is selling SVC 12" TC drivers for ~160 shipped :
Xmax of 1.4"
I modeled this driver vented in 2.3^ft tuned to 21.5Hz and it looked like it could be very nice in-room. ~9db down at 20Hz. Vent length would require a bend.