OK...I've ditched my mono cartridge

I have a Zero and a higher end Koetsu. Also, an arm that allows a moderately easy change.

The Zero is fine, but it lacks the ultimate refinement of the Koetsu, on EX mono records from the 60's.

I have done quite a bit of research and thinking on this subject.  Before you can dismiss mono cartridges as a class, I think you have to consider how the particular mono cartridge(s) you are evaluating was constructed. Only a few were constructed so as to be unable to respond to vertical motion of the cantilever/stylus.  One example of such are the Miyajima mono cartridges, which were designed to be mono in the first place.  (TSD and Ortofon may also make "true" mono cartridges.) Almost all other mono cartridges are stereo cartridges that have either been modified internally such that the capacity to respond to vertical displacement is reduced (and then the two channels are bridged at the output) or are merely stereo cartridges where the two channels are bridged at the output.  (Art Dudley reviewed 3 mono cartridges in one article wherein he tested each for its capacity to reject noise due to vertical motion of the stylus; the 3 were very different from one another in this capacity.) Obviously these 3 basic approaches can make quite a difference in the degree to which a given mono cartridge can be superior to using the mono switch on one's preamplifier.  This is why, if I buy any mono cartridge, it will probably be a Miyajima, but then I would still use my mono switch.  Unfortunately, the preamp driving one of my two systems lacks any mono switch (Atma-sphere MP1).  I am thinking of replacing the function of its "phase" switch with a "mono" function. Miyajima or no Miyajima, I would use BOTH the mono switch and a mono cartridge, optimally. (Have you tried that, Henry?)

But Henry, you did give the impression that you were comparing the dirt cheap AT mono to some very expensive stereo cartridges, although I don't believe you named them specifically. Meantime, I am quite pleased with the effect of using the mono switch that is available on my second system; it's such a big improvement over playing mono LPs in stereo that I don't play mono LPs on my main (no mono) system any longer.

I like Lewm's thinking, and yes, the Myajima is a fine mono cartridge.

I’m coming to think that, playing "mono a mono" can be very good, but equally important is the stylus profile and general design of the cartridge - irrespective of whether it’s a mono or stereo design.

I think we have two things at play with mono cartridges. The obvious one being that it’s a true mono design (including rejection of vertical motion). The second factor that gets overlooked is that the stylus profile is more suited to the groove profile cut by a mono cutting head.

Taken another way, if "limited" to two varieties of cartridge (i.e. a dual tonearm rig), I’m more inclined to run a vintage stereo cartridge (i.e. Ortofon SPU) in the second position over a mono cartridge. I think this ultimately provides the flexibility to play more of your record collection. 

This isn't to dismiss mono by any means ... just a slightly modified approach, and of course, if that second arm has a removable headshell, then one is free to collect a stable of easily swappable cartridges.

Cheers,
Thom @ Galibier Design

Everyone should reach their own conclusion.  My experience is that a mono reissue will sound different than a pre 1964 mono pressing, even when using the same cartridge (be it mono or stereo).  Mono cartridges will sound different, based upon construction.  A true mono cartridge generates signal from horizontal motion, only.  A stereo cartridge modified to become a mono cartridge will still respond to vertical motion.  Why is this an important distinction ?  Even though a modified cartridge is supposed to suppress vertical signal by summing, bridging, coil alignment, etc it is still there.  Unfortunately the existence of the vertical signal introduces phase anomalies that are audible.   It is easier and cheaper to adapt a stereo cartridge than to construct a true mono cartridge, which why there are very few.  At the low end there is the Denon DL102.  Anything better costs considerably more.  Some listeners swear by vintage Fairchild and ESL mono cartridges, that are more than 50yrs old.  However there are a few specialists that can rebuild them.