You guys are right-on, thank you. I ended up getting a $100 subwoofer on craigslist (200W Energy S10.3) just to play around with, and equalizing it with REW via computer soundcard (EqualizerAPO) made me realize that flatness, as you guys stated, is better off parametrically-EQ'd and the non-EQ response of the sub doesn't matter much. With low-frequency capability being determined primarily by maximum driver excursion (expensive) and surface area (big) of the driver (and low-tuned resonance if applicable i.e. port (big) or passive radiator (expensive)), you have to choose expensive or big, as you guys also stated. For instance the Energy S10.3 supposedly has unusually-low-freq capabilities for its price, but that 10" driver is on a 2.5 cubic-foot ported box (wonder what freq the box is tuned to?).
For an outboard / standalone DSP, is it really as simple as buying a $90 miniDSP (or better recommendation?) and uploading the REW coefficients? The craigslist purchase came with a crap budget soundbar. REW did a really good job of matching the SPL between the sub and soundbar despite the sub not having a volume/gain knob, would be nice to get parametric EQ without requiring my laptop.