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

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...

@tinear123 --

From what you describe above, and also given your prior experience with a more typical hifi-segment of direct radiating subwoofers (in conjunction with your current main speakers) there’s no doubt in my mind you need to go with a pair of horn subs (or other efficient pro-style large-driver (18"+) iterations). These are likely to complement your all-horn La Scala’s the best way possible, and will give you the sufficient headroom, power and way-of-delivery needed here.

DIY horn sub solutions, if you’re up for the challenge, sport a variety of options, like the builds from "lilmike" and "Ricci" over at the Avsforum (mostly tapped horns), as well as Bill Fitz Maurice’s THT sub and other varieties of his (mostly FLH’s). Pre-assembled iterations could include the tapped horns of Danley Sound Labs - TH-50, TH-115 and TH-118(XL) in particular. The latter two options here don’t extend notable below some 30-35Hz, but are more efficient (at least 105dB’s). The TH-50, fitted with a 15" driver, gets you 20Hz (at 97dB’s efficiency) to also excel with Home Theater duties and most any music material, while obliterating most any direct radiating alternative you’d care to find. And then imagine what a pair of them can achieve (only 4 of these were used in a very large IMAX venue in Chicago, just to give you an idea..).

I’m having a pair of lilmike’s MicroWrecker tapped horns (also w/15" drivers) build for my home setup, and they are closely inspired by the DLS TH-50 just mentioned above, with the same tuning frequency (22Hz) and efficiency. They take up 20 cubic feet per horn, and are to augment all-horn main speakers not that distantly related to your La Scala’s. The force multiplier of a suitable 15" in such a tapped horn is about factor 3, so the equal to 2x18" direct radiating units. Even so the tapped horn sound will likely be felt more viscerally and effortless by comparison.

Why horn subs, not least a pair? Because your main speakers are horns, all-horns at that, and because a pair (or more) will give you a smoother distribution of bass. In reality this is less about obliterating power than integration, coherency and natural (musical) presentation, but the former trait (i.e.: power in abundance) is not trivial. Most audiophiles are more or less conditioned to believe that more SPL/air radiation area "than needed" is excess baggage, so to speak, but it’s the whole point in the effort to attain an effortless and more natural presentation.