I try to go to as many classical guitar performances as possible, and the adjustable height padded piano bench is what I see most often being used. The fact that they are available at most classical music venues probably has a lot to do with it, but the point is that they work for professionals and conservatory students in the most demanding situations. Obviously, they work was well for pianists, who do just as much sitting.
I see the Wenger musician chair a lot, too (or possibly copies?) and I've seen a few of The Original Guitar Chairs on stage.
Until I started using a guitar support, I practiced using a standard wood chair and a wedge shaped cushion made for drivers that I found on Amazon. It has a layer of memory foam bonded to a wedge of dense, hard foam, and a grey suede-like cover. The shape compensates for the typical backward slope of the chair seat. The bottom layer of foam was so dense that I never felt the wood, yet I felt firmly supported.
After I started using a guitar support, I had to find a lower seat, and found a cheap x-frame keyboard bench that's sold under many brand names. It has only three height positions, but it's the only one I found that goes as low as 15 inches. So far, it's been adequate, though not as comfortable as the cushion. Some distant day, given enough time and ambition, I imagine reupholstering the bench using parts of the wedge cushion. (Hm, I wonder now if it might be simpler to cut down the legs on that kitchen chair...)