I finished construction of my DIY Dayton Ultimax tonight (save for the grille). It's in ~12 cuft, slot ported, tuned to ~17.5hz. Power is from a Behringer Eurocom AX6220 which, realistically, should put out somewhere around 1500w bridged with the driver's 4ohm load. WinISD models show it hitting 115-117db at the listening position with anechoic extension to ~16.5hz... so possibly ~14-15hz in-room. A quick function test yielded EXTREMELY positive results with a few test scenes (Avengers: Endgame final battle in Atmos on UHD, Independence Day first attack in AC3 on LD) with the HPF set to 20hz (as low as I could go via the Amp DSP's onboard controls) and levels rough-matched by ear. I'll hopefully get it slid into place start to dial everything in with the Behringer PEQ, REW, and Dayton UMM-6 calibration mic sometime tomorrow.
It's not much to look at as-is, but once it's slid into its dormer (front flush with walls on each side and media shelf / projector hush box above) and its grille is contructed/attached, it will basically disappear:
This will be its final location with my old DIY Ascendance Avalanche in ~6 cuft tuned to ~18hz powered by a BASH 300w plate amp still in place: