I had and still do have some headphones made by a company called Koss. Bought them in the 70's
They are called Phase4, ambiance expanding head phones.
They weighed a ton as they are cast aluminium and have oil filled ear cushions. They split the L+R channels so that half the left sound was played through the left side and the other half through the right side and visa versa.
Listen to some Rick Wakeman, Pink Floyd or even classical stuff with your eyes closed and the sound appears to be coming through the top of your head and puts you right in the middle of the stage with the band.
There is a switch on the side which turns this gizmo off so they perform just like regular headphones. The difference is amazing.
