How are we wiring our Mono systems?

There are plenty of people who want to play mono, even when the source is a stereo recording, meaning that the two channels have to be summed.  The easiest way is with a Y connecter, which is essentially what your mono button does if your component has one.  But, that is not the best sounding approach.  Combining the signals via a transformer is much better sounding—fuller sounding.  I heard this demonstrated via a box with switching and four or five different transformers, as well as a simple Y connector.  There were subtle differences between transformers, but a huge difference with the Y connector which was dramatically worse sounding than all of the alternatives. 

lewm,

I believe the dealer had it between the linestage and the amplifier.  He had a very young tech who built the comparator box and would also make the mono adapters for customers (put them in various vintage component cases with RCA jacks).  I don't recall what specific transformers were used, but some were current production and others were vintage; the most expensive being Western Electric transformers.   Based on recommendations he found on the internet, he built one such mono adaptors using two transformers (both Western Electric transformers) and that was the best sounding, but a bit pricey. 

I have no idea why transformers with zero gain sound better than a Y connector, although I sort of have a theory.  I suspect that the transformers introduce some phase shifting that actually improves the sound.  

You know a lot more than I do about this stuff; I just sat there and listened while someone flipped the switch on the comparator box.  There were two transformers that were very good and quite close in sound--one was the somewhat expensive Western Electric transformer, the other a much cheaper modern transformer--which made the cheaper transformer a "bargain."  I am not a mono system person myself, but I do see how it is more practical in certain situations where someone cannot sit in the ideal spot for stereo listening but wants good sound.  One of the buyers of the conversion box has a crazy nice mono setup built into his kitchen where a lot of time is spent.  He has a three way system with a field coil woofer, a Western Electric 713b midrange compression driver (my personal favorite compression midrange driver), Western Electric 32a horn, and an RP302 tweeter.  

Mastering for mono is not the same as just summing a stereo mix, so mono versions of albums can sound dramatically different from the stereo version.  In the early days of stereo, it was often the case that the mono version sounded much better--it took some time before engineers mastered the art of stereo mastering.  For example, in some respects the Beatles mono versions sound better than the stereo versions and that is even more decisively the case with some Cream albums.  I have some Cream CDs that have both stereo and mono versions, so one can make an instantaneous comparison, and the mono sounds better.  

Turning a stereo mix into a composite mono version is truly a crapshoot, particularly with more recent recordings.  No one masters recordings these days with any consideration of how it will sound when played on a mono set up.  Still, the sound can be surprisingly decent.