@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.
