I had some time today to play with the mono/stereo button, as it is to hot to get anything done outside.
With a copy of a classic records 200gram copy of Griffin's A Blowin' Session, things sound very similar, the only minor difference I could note was that Blakey's cymbols sounded just a tiny wee-bit more clear in stereo.
With my cleanest vintage blue note mono, a copy of Bobby Hutcheson's Happenings I could not detect any difference at all.
With a black label Atlantic of The Jimmy Giuffre Clarinet, it seemed to help to deaden some of the light hiss in this pressing.
With stereo recordings it obviously makes a difference and I have used it when I am particularly annoyed by things being too left and right, if you know what I mean.
It may make more of a difference if you have a nicer system.
My TT is a beogram tx2 with a new smmc4 stylus - reciever/amp is a late 70's yamaha cr840 - speakers are 2 pairs of polk-audio s4's.
I think I'll just keep it in stereo for the bulk of my mono listening and just use it for what sounds like bad stereo to my ears.
Bill