I think its something to do with microphone placement and the tube equipment they used. I also run esl speakers, and have found many beautiful mono records have so much depth to the sound stage you can fool many people into thinking it's stereo. Makes me understand the mono purists' point of view, especially when you hear the 80's records, with so many microphones and huge mixing boards the stereo effect is lost. A great stereo record can produce a super wide sound stage, but most stereo records are not engineered well enough to do so. Ray Charles insisted on five microphones at all times, even with big band, piano and Raylettes. He had golden ears.