I was reading up on the Steve Hoffman forums about listening to mono vinyl through a stereo cartridge. He personally advises that in the absence of an actual mono cartridge, that you should use a summing cable to combine the two channels together if you don't have a mono button in the preamp.
The summing cables would basically be a set of in and out Y-connectors. The first Y takes the left and right output signal and sums them together into a single signal. The second Y splits that summed signal apart into two and you feed those two into left and right inputs in the destination. The advice I read is that this should be done after the phono preamp stage.
The rationale is that mono records are actually dual mono, and that there can be some phase distortion between the two channels, in particular noise. By crossing the signals together the phase noise gets eliminated. People are reporting experiencing hearing much less surface noise in their mono records when doing this.