My sub quest began ca. 1979 when I acquired Magnepan MG-I's and decided they really needed some added bass. There were only a few around and that time, mainly AudioPro (long gone), M&K, and Velodyne. Velodyne had an interesting servo design but was way too expensive for a college kid, even one with dealer's accommodation (I was working as a tech in a couple of stereo stores). So I built my own version, using a 12" dual voice coil woofer from Infinity's IRS and a DIY crossover and servo circuit, and built a box to house the driver based on the usual Helmholtz equations (did not have speaker design SW back then). I used the second voice coil as the sensing circuit for the servo, an approach that was easier (and cheaper) for me to implement than Velodyne's approach back then, and with a few advantages like better control of thermal effects. The control box could drive a single amp or bridge a pair of channels; I used it with a Phase Linear 700 but switched to a bridged Hafler DH-200 I was not using at the time. It was better than most passive subs, and equal to or better than the Velodyne's we had on hand to compare (measured and listening tests).
Through the years I owned a number of other subs, mostly cheaper models from Mirage, Infinity (to match with my Alpha's, not the IRS2's I owned much earlier), BiC, a few others I've forgotten. Along the way I built a few other DIY models, mainly passive, for friends, experimenting with larger subs in (much) larger boxes as well as some arrays of smaller drivers (despite all the hype about "faster" the arrays did not work well for me). I did not own but built a few open-baffle subs for friends and customers, some servo, some not. They usually had higher distortion though the servo helped. For the record, a well-implemented passive sub was comparable to my servo design, but usually in a larger box and using a much larger (and more expensive) amplifier.
I took a hiatus, Life and Work happened, and finally returned to setting up my main system about ten years ago. My old sub's driver had been holed by a moving company so was no longer functional (they reimbursed the cost by the driver was unobtanium by then). Having funds for a new sub and zero time to build one again I auditioned a few. I heard a couple of B&W's and did not like them (they have come out with much improved designs since but I've not heard the new versions), JL, Paradigm, Velodyne, and one or two others I have forgotten. I did try a REL but did not care for it as it was configured (using high-level inputs from the main amp, not a configuration I care for). I heard about Rythmik, spoke some with Brian (who it turns out has a similar day job and similar servo circuit), and order a Rythmik sub . I was not able to do much in the way of listening tests since I did not have them all on hand, but I tried my best. I thought the little Rythmik as the equal of the 12" and 15" mid-level JL, Paradigm, and Velodyne models, mostly on par with their upper (and thus 3x~5x more expensive) models. I also decided that, in the perhaps $2k to $5k range of commercial offerings, subs had significantly improved from those early days. (Aside: I did not hear of Seaton until later, but would expect them to perform superbly as well). The HT revolution had clearly expanded the range and performance of subwoofers.
For myself, I stuck with Rythmik, buying a few more over the years, and quit auditioning others for lack of time and interest. My experience suggests there are many really good options these days, and a few clunkers, but with so much Internet chatter and more reviews (with measurements) it is easier than ever to find a good sub.
As an aside, the "speed" thing puzzled me, but I decided many years ago what really mattered was low distortion, even frequency response (which the room will destroy anyway), and sufficient cone (driver) control to ensure the sub "starts" and much more importantly "stops" quickly and cleanly. A sub that "rings" after the source (signal) has stopped will muddy the bass. While I chose servo for it's excellent performance in a small size, a good non-servo design can be equally as effective IME, just takes more care in integrating driver, amplifier, and box as well as consumer choice so the sub can provide the sound levels required without distortion. Since distortion occurs at harmonics of the fundamental, it is usually much easier to hear the distortion than he fundamental when we're talking subwoofer frequencies. This makes lesser subs sound "richer" or "fuller" but they are not "cleaner". A good sub is a revelation in how the deepest bass supports the overall sound without adding extra "boom" that is not there.
All IMO - Don