It's such a specialist area. Horns are incredibly efficient so you can run them on a couple of watts. So they appeal to owners of "wimpy" valve amplifiers.
From memory they can sound coloured. But don't quote me. Some progress must have been made since I last read anything about them. Colouration is often quickly forgotten once you have listened to something for a short while.
Horns are not well known for offering a great deal of bass except in enormous sizes. They are supposed to offer considerable dynamic range and are excellent for microdetail and micro dynamics. (Small variations in loudness usually lost on other types of enclosure.
You obviously have some experience of the type from your parents own horn enclosures. The obvious question is why you can't have their complete loudspeakers? Are they upgrading their drivers so their enclosures are not available? Or do they not suit your situation? Too large? They are obsolete? Woodworm?
Horns are a fascinating subject. Their owners and builders can be quite fanatical. I would try some gentle browsing to get a better feel for what is happening these days.
It's a shame you didn't mention Lowther or horns in your thread title. You might have woken a slumbering horn enthusiast.
Perhaps the mods could be persuaded to change the heading? Or you could simply start a new thread: Lowther Horns: Any good?
If you like the sound of your parents' Lowthers. Then you should already know whether they are any good.
If you do go ahead and have the enclosures built do post some pictures. It will be fun to see what is involved.
It will also make a change from the typical "Which 7.1 speaker package is best for £75?" threads.

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