Yes iv seen this one being mentioned 100s of times on the forums---and yes I know you guys made 1., Pro Audio Technology makes a 24 inch, I have heard that and a 21inch many times and always found them to sound slow. Ultimately the Quested 18 inches were by far one of the best and punched well higher than most 21 inch could. The larger the driver becomes the harder it tends to be for the designer to make them sound fast punchy and articulate.
Sounds like you experienced poor designs, or subs that were driven too high above their design range.
Subs aren't supposed to sound "fast, punchy, articulate." They are supposed to move air and typically at frequencies well below 100Hz.
Woofers need to be punchy and articulate, and that is accomplished by proper enclosure loading, good driver linearity, and clean power to drive them them.
I had a pair of 6 cubic foot I. M. Fried MLTL's driven by 8 inch woofers in the 80's that were rated to go 114 dB at 17 Hz (and I believe it). Way too much output for residential spaces, but so musical! But brought tears to my eyes in a 160 seat, 20,000 cubic foot chapel. In comparison those TL's would be a much better fit for a small arena.
Actually I'm wondering if a tiny highly affordable TL subwoofer is available that can do just 100 dB at 20 Hz, or am I asking the laws of physics to bend too far?
All you need to do is specify the size of sub you want and I'll find a driver to match that criteria. But if you want small at all costs, your best bet is to go with a Passive Radiator and throw a driver in the enclosure that has a lot of linear throw and can soak up lots of power.
The problem with small enclosure and lots of dB is that the air movement can cause audible turbulence at the terminus of the line (or vent in a Helmholtz radiator). T-lines tend to do a little better at reducing that port turbulence than typical ported enclosures, but a proper PR design works best of all.