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

There are a lot of great options for "musical" subwoofers.

What size is your room?  What's your budget?  Do you have a preference in terms of connections (speaker/RCA/XLR)?  Music only or music + movies?  Any placement / WAF issues?

Ported subs can be as good in a music only system, but my first choice would be a sealed sub(s) for music.  Rythmik's servo controlled subs are great for music.  Without knowing more it's hard to say which one would be best.  I have two SVS SB13 Ultra subs in my main music system and am very happy with those and wouldn't hesitate to recommend SVS.  I have two Rythmik F25 subs that I use for home theater.  I would use those (or maybe just one) for music, but I can't due to space/placement issues in my room.  They are too big to fit where I would want them for music.

Fast and small, or like a fretless bass player, who wants speed and nimble in the box. That would be multiple or single 10’s in a single box. You need drivers that can go low, but still have a high acceleration factor (it’s a driver spec). This will not happen with 12" drivers and can barely happen with 10’s.

When I wanted to do stereo with subs -right, and have subs that could work with ANY two way and match up as seamlessly as possible, I went with TWO (one per side) of the Velodyne optimum 10 subs.

High powered, matched to the individual main speaker locations, sealed, nimble, and fast enough. With high enough output. Closest match for the ’best attempt’ I could find on the market, at the time -and available to me. Still regret selling them.

So, some set of "known for their quality of articulation" 10" high powered subs (2 required) is a good place to start looking. 12" is too large, and slow.... 8" too small and struggle to get the tunes out, so you loose dynamic articulation and have a smaller dynamic window of linearity in all areas,...where the klipsh speakers definitely do not suffer dynamic issues.

12" subs would be a better match for dealing with high bass levels for ~full~ dynamic capacity matching with the klipsch speakers, yes, but speed, no. Articulation matching up, no.

A pair of tens for the attempt, it is. This is likely the closest for obtaining satisfaction --for the very vast number of times you turn the system on.

Forget ported subs, leave that to home theater which is about pushing the sub as hard as can be, and articulation is down on the list of requirements, most times.
Importantly, when Doing this, i was only interested in leaving the mains speakers, what ever they might end up being, and being changed out many times..to have them be unhindered and running full rage with no signal degradation from external crossovers or anything.

This means the subs carried all the matching up work. so the given 10" sealed subs had to do all the heavy lifting in the matching up part. maybe multiple 12" in a sealed box. Two subs like that. Under driven subs  can go a long way to help keeping dynamics clean, delineated, and nimble. that's big and expensive (think JL fathom, gotham, etc) and was not my intention at the time.

You are getting into the area of ’the impossible match’ Horn loaded dynamic and fast bass, and trying to couple that with subs issue.

Which is exactly where you are.... Tricky. Tricky at best ...and money and intelligence needs to be wisely paired and thrown at it.

You almost require multiple fast pro sub drivers (light cones, massively efficient) in an infinite baffle in wall design, in order to match up. But that gives timing issues re matching up.

You are reaching for the last bit of infinity which troubles you by demanding infinity in return.

From EQ’g many a kick drum and I mean many, I have never needed to mess below 40 HZ
i would add some meat at 50 HZ
overly hyper flat bass EQ rarely sounds good....

have fun

i used a stacked pair of LaScala for my PA in a Punk/Rockabilly band...