My experience: My first subwoofer was a Def Tech PF15. This is a typical, boomy, sealed design. Although I lacked funds to buy an expensive DSP device, I did invest in a Behringer FBDP 1124P parametric digital EQ. Following instructions I found on the web, a test DVD with frequency sweeps (Avia Guide to Home Theater), and a Radio Shack SPL meter, I did a good job of smoothing out the response. It is time consuming, especially if you use a computer to determine your level adjustments, which I did. In order to increase the gain for the sub, I also added a Paradigm PDR-30 sub controller, which adds 14 dB of gain and, importantly, a phase control (no phase control on the PF 15). While the results were an improvement, these adjustments could not make the PF15 "faster" or more "musical." The peaks were now reduced, but I cannot hear bass below about 26 Hz, even though the SPL meter indicates solid output to 20Hz. Could be my room or my ears; not sure.
I knew better bass was out there to be had, and I waited patiently for local A'gon sellers to offer a pair of Vandersteen V2q subs. I figured they'd mate well with my Vandy 1C mains. These are very unconventional in design. Visit the Vandy site - www.vandersteen.com - for details. Richard Vandersteen told me personally that these subs are designed to have a pretty flat in-room response to 22Hz when placed in the room's corners. Also noteworthy is the requirement to determine your amp's impedance. Using the *temporary* adjustable outboard x-over, you determine the optimal crossover settings. Once determined, you must order in-line x-overs from a Vandersteen dealer (~$125/pr). The temporary x-over is NOT transparent. I procrastinated in ordering the permanent filters. That was a mistake. With the in-line filters, the sound, especially the stereo imaging, improved dramatically. Without any signal processing (aside from the inline filters), the bass I get now is tight, deep, tuneful and accurate. That last point is very important. Unlike some subs, these dissappear unless they have a signal to reproduce. Then, you hear the bass, but not as booms from a subwoofer. The bass integrates perfectly with the mains. There is no sense of a subwoofer per se, but only of the mains reaching really deep for bass you can feel. Note that this is only the case when the source material contains deep bass, which is, IMO, as it should be.
These subs list for $1400 each, plus the filters, but can be found used for under $900 here on the 'gon from time to time.
I feel this is a clear case of good design trumping aftermarket fixes and tweeks. YMMV
Oh, the Def Tech? It's still there, doing LFE and bass duties for the center/surround/back channels of my combo 2-channel/HT rig.