Mono recording question...

The MONO switch on a preamp adds the two input channels together and outputs this result to both channels. This will cancel any signal that is out of phase in the two input channels. This is a good thing to do when, for example, playing a Mono LP where there may be a lot of out of phase aignal, but it's all noise, most notably rumble.

For a Mono CD, the Mono master tape was probably digitized once, and the same digital file put on both CD channels. This means that there will be no out-of-phase signal between channels, and the Mono switch will do nothing.

Or, if the mono tracks are out of phase, to any degree, you might get some loss of your high frequencies. As Newbee said, give the mono switch a flick, and YOU decide! It might help to have someone else do the "flicking", so that you can have a real A/B comparison.

Not a stupid question, but why don't you just flick the switch and find out? It may depend on the manner in which the mono signal was transferred to the CD. It might cancel out some noise on the CD from the remastering.