Guys: Let me chime in, since I have done a fair amount of work in this area (smile).
Even if the LP groove is mono, you will get maximum information retrieval if the vertical stylus contact is maximized and the longitudinal stylus contact is minimized. This dictates a line-contact stylus - similar to the requirements for a stereo LP.
But there are two things to watch out for regarding the stylus shape.
One is that many earlier mono LPs have shallow bottoms, and/or the bottoms of the groove are mired in decades of accumulated grime. You can run into problems with tracking and noise if the stylus shape is narrow and pointed enough to allow the very tip to touch the bottom of the groove or the dirt that may be there.
Two is that a line-contact stylus will make the effects of stylus rake angle (SRA) and cantilever rake angle (which I think is a far more descriptive term for cartridges than VTA) more noticeable, and if you have a tonearm that doesn't allow easy height control, you may be better off with a spherical stylus.
Regarding coil structure, it should be real mono rather than strapped stereo. On paper, strapped stereo coils, or using a mono switch on the preamp will get the job done. In the real world the results are audibly better with real mono coils. Canceling whatever vertical noise component that may be present in a mono groove assumes very tight matching of channel output as well as magnetic and capacitive crosstalk - but this assumption doesn't hold up well in the real world. And in general (with amplifiers as well as transducers), not picking up an error component in the first place is preferable to picking it up and trying to cancel it out later.
Regarding vertical compliance, a mono cartridge should have it. Mono LPs are not tougher than stereo LPs, both are made from the same materials, and both will remain in good condition for a lot longer if grooves are not subjected to high pressure - especially if that pressure is concentrated on a narrow area of the groove. If the cartridge suspension has no vertical compliance, the same stylus will require higher tracking forces than if there is vertical compliance.
This also becomes an argument in favor of line-contact styli, since they do the best job of distributing the vertical tracking forces over a wide area of the groove.
Regarding costs, the Dorian Mono was never US $1600 to my knowledge. We do not have any authority to dictate the retail pricing in a given country, but I would be surprised if the US retail cost for the Dorian Mono ever exceeded $1200.
Having designed and produced the Dorian Mono, Helikon Mono and Titan Mono, I can say that the market for mono cartridges is very small. OTOH a mono cartridge requires a different set of parts from a stereo cartridge, different work operations, and the sonic tuning is different. IOW adding a mono cartridge in a manufacturer's product lineup will require its own dedicated design and development program, while making overall production efficiency decidedly worse. There will be some increase in component cost for a mono cartridge, because designing and ordering custom-made components in small quantities inevitably results in higher per-piece costs, but that is not the key issue.
Having made the Dorian Mono, I am somewhat doubtful about the business wisdom of making a Delos Mono, but if we were ever to do so, it would have to be produced perhaps twice, at the most 4 times a year. If we were to set up dedicated production batches for a Delos Mono, and make at least 50 cartridges per batch, m-a-y-b-e the development program and the production effort could be justified.
For the time being, the Kleos Mono is our lowest-cost mono cartridge.
However, we will continue to service, readjust and rebuild all of our previous and present mono cartridges, including the Dorian Mono, Helikon Mono and Titan mono (we even built a very small number of Olympos Monos).
hope that this has been informative, jonathan carr
