A perfect speaker would reproduce some of the most important characteristics (to me) that I hear in live instruments: timbral complexity, richness, full, with dense, palpable imaging and presence. And yet with a sort of combination of clarity and ease. In fact, Joseph Audio actually gives a good description in terms of what he is trying to reproduce:
"Live, unamplified music has unmistakable presence and clarity. Yet, at the same time it also sounds relaxed and warm."
That's exactly as I hear it. And funny enough...I find my Joseph speakers get about as close as I've heard to these qualities, especially with my CJ tube amps.
Another property the perfect speakers would produce is "surprisingness." Whenever I close my eyes and listen to real voices and instruments, the range of timbre and sound seems sort of unlimited. Whereas every sound system homogenizes things to a great degree. Once I hear a few tracks with drum cymbals, or sax, or acoustic guitar or whatever, I pick up the particular timbre of that speaker and I know what those instruments are going to sound like forever more on that system.
Again, one reason why I actually became infatuated with my Joseph speakers is because they offer more "surprisingness" than I've ever heard in that regard (with the possible exception of the MBL omnis I owned).