Why not horns?

Can someone provide some general impressions regarding the difference between a horn bass, folded horn and a good modern sub woofer(s)?

Is the difference more sonic qualities, room loading or is it that only a horn can keep up with a horn?

Macrojack, the folded horn you asked about is one of a thousand variations on the same idea. I can't speak to that particular model but as a general design it works pretty well if done right but has a few problems. You have parallel side walls which means you have resonant frequencies and it really doesn't flow like a what I would call a true horn. If you look at mine it is expanding in all directions as you go from driver to mouth so no parallel walls and no sharp bends like in a folded design. However, the folded horn is a good compromise.

Lokie, as you realize it is difficult to describe sound but the best I can think of is a box is muddy compared to a horn. Not that good subs sound muddy but horns are just a bit better. I think it is usually described as the horn is faster but I'm not sure what that exactly means. If you could hear one you would understand but unfortunately they are few and far between.

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I love my Sansui sp-5500's MADE IN JAPAN! I had all the wires and crossover parts CRYOED

I'll chime in regarding Klipschorns.
I had never heard nor seen a pair but decided to look to buy after becoming somewhat bored with my system over the past several years. I had Martin Logans and before that a couple of models of Maggies, Paradigms before that. I just thought it would be fun to try something completely different. I sold all my stuff, bought the KHorns the day I went to listen about a year ago and have been enjoying them since. I have a few SE amps to drive them. Sound is at least as good as what I had before, and even a bit cheaper ($2K for the KHorns, with less expensive amps).
KHorns (or horns in general) might not be for everybody, but they do work well. I'm sure I'll be playing Hi efficiency speakers and low power SE amps for a long time.

You do not need 35ft long horns. Herman I also have front loaded bass horn with 16ft horn path. You also have to consider the room if rooms right the bass horn can couple to floor ceiling to produce a even larger horn. I do this with all my horn designs.