Single driver planar speakers are among. the most phase coherent available, the Orchid monitors which came from a recording studio heritage used extra drivers to create error correction. These sounded very accurate, in fact I did once emntemr discussions with the designer to market them. I did design a couple of phase coherent monitors using time correction back in the day. In those days it was all analouge, hese days it's much easier to do it in software. I find that old Tannoys with suitably modded x-overs are very good in terms of time and phase accuracy.