Ambient loops

Use a system set up with a rear ambient loop if you want to hear 3D sound as you ears normally hear. You will need an old Dynaco Quadapter QD1 if you want the load on your amp to be somewhat close to normal.  Even better, is to use a separate amp for the rear loop.  All stereo recordings, going back into the 60's and 70's, have an ambience that gets recorded, but can't be played back by the front speakers. You cannot ask the front speakers to play both the direct signal AND the ambient signal at the same time - won't work, yet at the same time, there's this ambient signal that gets recorded just the same, but you need an ambient loop setup in the rear to extract it. The two speakers up front can't reproduce the ambience, and they're also in the wrong place to do so.
This works for stereo amps and I have tried it as an interesting experiment: as Brian Eno suggested back in the 70s, one can place a small speaker behind the listener and connect it to the two positive terminals of the amplifier for a "third" channel effect. The resulting sound is the differential between the two channels. It won't harm the amplifier and places no additional load on the amp. One can put in-line a potentiometer to adjust volume of needed.

The results are interesting and do expand the sound field greatly.