Anyone have 'stereo blend' control experience?

I'm interested in any details anyone may be able to provide concerning 'stereo blend' controls, for progressivly reducing a 2-channel signal down from full separation towards a mono blend, presumably through cross-mixing.

What older equipment might you be familiar with that featured such a contol? Did any incorporate a 'defeat' provision, or was the control always in the circuit? If you ever used one, how well do you remember it working?

Do you have any electrical design knowledge of previous implementations of this idea, or suggestions on how to make one? What kinds of parts were used in any older designs you may have seen? Do you know of any schematics?

I realize this probably was a pretty obscure feature even in its day, but maybe some of you more, ahem, experienced hands out there know what it is I'm talking about. TIA for any info!