Chair or Sofa? Which do you use?

Sofa, because of the significant other(s)......

If your intention is romance, even occasionally, go with the sofa. Otherwise, chairs are the way to go as long as they are comfortable. It helps if they are also easy to move so you can share the sweet spot with a friend. While listening alone, you can claim the prime spot yourself. With chairs, you will be able to sit more comfortably very close to a friend so you can both better enjoy the music.

Sonically, I don't think it makes much difference. Sitting exactly in the sweet spot typically does. This is especially true (and rewarding) with narrowly focused speakers such as the Dunlavys.