Single driver speakers that sound boxless. These can be fostexs or lowthers with tubes or (ideally) full-range, non-hybrid electrostats with tubes. Less ambitious boxes and crossovers can be bad. As a general rule, it may be best to avoid them. I suggest electrostats for human voice with the caveat for human voice and preferably no electronic instruments. Electrostats fare best with acoustic sounds. YMMV.
