Lew, I should have posed it as a question rather than a statement. As in: "Is a mono switch the same as a mono cartridge?" I ask because the stereo pickup creates it’s output signal from both the horizontal (monaural) and vertical (stereo) modulations. When the stereo pickup plays the groove of a mono LP, the signal resulting from the cartridge trying to read a vertical modulation (of which there is none on a mono LP, correct?) contains any noise and/or distortion obtained or created by that attempt---it is now an intrinsic part of the signal. Does putting the Mode switch to it’s mono setting enable the pre to cancel that noise? I don’t know.
A true mono cartridge, in contrast, does not try to create a signal from the vertical modulation to begin with---it is deaf to the noise/distortion created by trying to do so. If it is done perfectly, a mono cartridge should sound the same whether the Mode switch is set to stereo or mono, I would think. Of course, perfection is an unachievable goal!
I posted this, only to see Lew's post directly above it. Never mind!