advice for mono cartridge

That’s valuable testimony. A dichotomy arises because if you switch cartridges then that brings into play the fact that you’re comparing two different cartridges. The mono cartridge might just be better than the stereo one. For each of us the outcome could be different. But you’ve stimulated me to do some serious comparing. One thing is for sure, mono LPs should be reproduced in mono if you want to get the most out of them.

After a year of playing The Beatles remastered mono LP set I bought in September 2014, I bought the Audio Technica AT-MONO3/LP HOMC (mono, of course). Amazon sells these cheaper than anywhere else--$118.53 with free shipping. It features a 0.6mil conical stylus. It is also "true mono" in that it only picks up the horizontal modulations. Dropping the needle on the record (a vertical contact) is dead quiet.

My tonearm accepts interchangeable headshells, so switching from stereo to mono is pretty quick.

It was a revelation. The remastered mono Beatles records were more focused, quieter, and dynamic. My enjoyment of these records jumped by 30%.

I soon discovered that I had some other mono remasters. One such is the Analogue Productions dual-45 rpm reissue of Nat King Cole's "After Midnight" abum from 1958. Another is "Drummer Man," a Verve mid-'50s pre-stereo hi-fi LP with Quincy Jones doing the arranging and conducting, and a reunion with Krupa's featured soloists, Anita O'Day and Roy Eldridge. 

I knew this record like the back of my hand, but with this AT mono cartridge, the playback "popped" as I'd never heard before.

Given the really high prices of some high end mono cartridges, the Audio Technica cartridge for $118.53 is a no-brainer, and moving coil to boot. I like the conical stylus as well. Much less hassle in alignment and setup, and the stylus pretty much tracks the same regardless of what part of the record surface it's tracking. The only thing that might be better would be a Shibata, but for $118 I'm not complaining.

Later, I retrieved some true vintage mono LPs recorded and pressed in the '50s and '60s, acquired from thrift shops for 99 cents each (one is even an original Everest recording/pressing of Mozart wind ensembles). Played on my stereo cartridge, they were too noisy; I originally played'em once and shelved them. With the AT-MONO3/LP I pulled them out again, and--Eureka!--the surface noise was gone! Just pure center-channel music. And some of these mono LPs (Roger Wagner Choral, Vince Guarldi's "A Boy Named Charlie Brown) were made in 1969! Who knew mono production was active that late--a full 11 years after the introduction of stereo LP.

It's a win-win all around--cleaner sound, better dynamics, focused center channel soundstage, and way less surface noise.

Not many mono experts here as i can see. 

Here is more about mono cartridges in the article by Art Dudley. Only the cheapest mono carts have conical tip for mono, most of the high-end mono have at least elliptical or line contact.

What i can see in Lyra Etna MONO specs is LineContact stylus:

"Lyra-designed long-footprint variable-radius line-contact nude diamond (3um x 70um profile, block dimensions 0.08 x 0.12 x 0.5 mm)"

Why High-End manufacturers of the modern Mono cartridges are using LineContact for Mono ?  Probably because LineContact is better for Mono ? 

Why do we need a mono switch if a true mono cartridge has just 2 pins with just two wires coming from the headshell (not 4). The generator of such cartridges is a true mono. It's not enoug for a proper mono playback? 

I agree it seems redundant to engage the mono button when the cartridge is mono.  However comparing the two with just a push of a button and my ears.  I prefer to use it.  Maybe a McIntosh preamp thing.