One sound quality I rarely see mentioned is how a speaker sounds all around the room. While the "listening" position is important, I also demand that speakers sound good as I walk around the room, and also around the house. I’ve had multi-way speakers whose sound falls apart as soon as you stand up.
My 15" Tannoy’s fill the room, and sound natural all around the room, and while I’m working in the kitchen. This is very important to me. I also use an 8" Tannoy driver as a center channel (in a DIY cab). It makes an ideal center, with no off-axis "phaseyness". No my Tannoys are not perfect, but this is a quality that they excel at, over non-concentric speakers.