Gondo, A mono phono stage adds yet another layer of complication, in terms of decision making. First, why use a mono phono stage if your "mono" cartridge is naught but a stereo cartridge that is internally strapped to produce a mono signal? In that case, one's "stereo" phono stage is merely processing the very same signal to produce two mono output signals, to drive two speakers in mono. I actually like that, because it produces a bigger more room-filling sound than I can achieve driving only one speaker of a stereo pair. Now, what might be the advantage of a mono phono stage? I guess it would be that the two stereo channels in your stereo phono stage are not exactly matched, thus producing subtly variant signals that end up as two slightly different signals on your two speakers, maybe robbing some ultimate fidelity. But if you worry about that, then why couldn't one simply abrogate the output of one channel of a stereo phono stage and thereby use only one half of it, to drive one speaker?
Do OMA suggest driving two speakers with the mono output of their mono phono stage, or just one? Probably the latter.
One could write several more paragraphs on the other things to consider, like for true vintage mono LPs not equalized for RIAA standards. In that case, if the OMA product has selectable equalization curves, therein would lie an added value. But it's a sticky wicket.
