I’d personally describe Frugal Horns as more of a short transmission-line or long flared port than a horn. In fact I really struggle with the whole term as to my mind they aren’t horns at all. Same goes with Lowthers etc. Horns are Klipschorns, La Scalas, Avant Garde etc!
Regardless they are interesting little speakers, but to my ears are vastly more at home on valve amps, including SETs, than any solid state I’ve heard. I’ve only heard them a few times, but certainly enough to form that opinion pretty firmly. Obviously there are many types too; lots of different drivers, and a couple of different cabinet sizes, so hard to generalise (I’ve heard several variants, but can’t recall which!). The problem to my ears is in the wrong context or location they can sound thin and mid-forward, as small single-driver speakers tend to, and I suspect a Nait may work against that rather than help ameliorate it. When working well in a synergistic system they can be very good indeed.