Sub recommendation to augment Klipsch La Scala's ???

Smooth bass = "fast" bass, because it is the peaks which decay slowly.  These peaks are inevitable and are caused by room interaction.  You can move the sub and re-arrange the peaks (and dips), but you cannot eliminate them by positioning alone.  The best you can hope for is to find the "least bad" position. 

One approach which works well is to use multiple small subwoofers distributed asymmetrically.  Each produces a different room-interaction peak-and-dip pattern, and the sum of these multiple dissimilar peak-and-dip patters is significantly smoother than any one alone.  Unlike EQing a single sub, this improved smoothness holds up throughout the room, rather than being confined to a small area with the response actually made worse elsewhere.

In the bass region, speakers + room = a minimum phase system, which means that the frequency response and time-domain response track one another.  Fix one and you have fixed the other.  So when we use a distributed multisub system to fix the frequency response, we have simultaneously fixed the time-domain response. Therefore, a good distributed multisub system is quite "fast", integrating well with systems like Klipsch Heritage series and Edgarhorns when setup correctly.

 This may be somewhat counter-intuitive, but the in-room smoothness (and therefore "speed") increases with the number of intelligently distributed bass sources.  So two subs are twice as smooth as one, and four subs are twice as smooth as two.  You can find anecdotal evidence of this trend in reading posts by Maggie and Quad owners (dipoles have inherently smoother in-room bass than monopoles and therefore sound "faster"):  Most Maggie and Quad owners who try a single sub end up ditching it and going back to no sub because the discrepancy sticks out like a sore thumb, while most Maggie and Quad owners who try two (or more) subs keep them. 

The ear/brain system actually has poor time-domain resolution at low frequencies, but very good loudness-domain resolution.  If you eyeball a set of equal-loudness curves, you will see that they bunch up south of 100 Hz, which means that a relatively small change in SPL sounds like a large change in loudness.  In fact a 5 dB change in SPL at 40 Hz sounds like as big a change as 10 dB at 1 kHz!  This is why making the effort to improve the smoothness in the bass region can give very large subjective benefits. 

It doesn't matter how fast a subwoofer starts out - by the time your brain begins to register the pitch of the bass note, the room's effects are all over it.  Sure there are audible differences between different subwoofers, but the biggest improvement arguably comes from solving the biggest problem, which is room interaction.  The elephant in the room IS the room.

Imo, ime, ymmv, etc.

Duke

dealer/manufacturer