A subwoofer offered from the Klipsch Heritage line would be an interesting addition. It’d definitely need to be horn-loaded in some variation (both with regard to the need for very high efficiency as well as the specific sonic imprinting of horn-loaded bass), with the only solution really being the front loaded horn.
Problem with that though is size; if, say 20-25Hz is desired here - and it would seem to be mandatory with at least ~25Hz extension for it to make any sense - a truncated 1/4 wave FLH with a restricted mouth area to save overall size will still be 18-20 cubic feet in volume per cab with a 15" woofer, with diminished (though still high) efficiency and limited bandwidth to boot, not to mention potential frequency response irregularities.
Being a non-truncated variant of a FLH, certainly insofar mouth area goes, will see a definite uptick in efficiency and better bandwidth upwards and overall FR smoothness - that is, at the expense of even bigger size with a similar tune. Sharp horn path bends with a folded horn is another matter wrt. upper bandwidth limitation, but is usually only an issue above ~150Hz with a full mouth area, so nothing to really worry about with subs usage. Air velocity build-up can be an issue with 180 degree horn path bends and path restrictions near the throat, but only below tune (so a high-pass filter is in place here) and at SPL’s that would see the typical audiophile run screaming from the scene.
So, a non-truncated (mouth-wise) 1/4 wave FLH with very high eff. (i.e.: ~105dB’s) and a tune no higher than 25Hz? That’s gonna be BIG - i.e.: in the vicinity of ~35-40cf. A truncated 1/4 wave FLH (95-100dB eff.) would be a relatively sound compromise size-wise at ~20cf. with a tune at or just below 25Hz, while still providing the type of horn-bass response that gels particularly well with Klipsch’s all-horn main speakers. Personally I’d go with a tapped horn vs. the truncated FLH (and, in some respects, the non-truncated version) for a variety of reasons, but that principle is patented by Danley Sound Labs commercially, so a no-go here.
My take is Klipsch may find a sub option in their Heritage series, certainly a horn variant, to be a tough sell, while also blurring the distinction and "hierarchy" of their model line. To me it’s strange learning how many an audiophile fuzz about the stated (lack of) extension of the La Scala’s, let alone Khorns in the face of their excellent bass reproduction with music as is (notwithstanding the "character" of the upper bass), but I can see why those who do cherish their bass quality would want an octave more to lavish. Having owned the Belles homage speakers from Simon Mears, the Uccello’s, I know the difference this can make.
[Horn sub implementation may seem complicated at first and practically less than doable in a domestic environment, and it certainly can be a challenge to get your head around; indeed, you have to know what you’re dealing with and be willing to accept what it takes (and not least that which you didn’t foresee), one way or the other, but it comes down to hearing for oneself the difference they can make, and then will their inclusion in one’s setup. I for one am not going back to direct radiation with subs.]