Why do some model names use "MKII"?

We have used this nomenclature. Our first lineup did not use the 'Mk I' tag; that got applied after the Mk IIs were introduced. So the Mk I lineup went for about 17 years.

The Mk II was the most significant update that we had developed during that time. In it, we went from miniature tubes to octal tubes, so to update older amplifiers required us to do a lot of chassis work to accommodate the larger tubes. None of the prior updates needed this- nor were those updates as significant- hence Mk II.

The Mk II series went for 9 years and in 2005 we introduced the Mk III series. What is nice about this approach is that it allows us continued refinement of the basic models, for example our flagship preamp, the MP-1, has been in production since 1989, although considerably updated since its introduction. This approach allows us to support the legacy products in the field- we can update any of our prior amps or preamps to meet the latest spec, and when we do that we reactivate the warranty.

I do trust products that are well sorted overtime by gradual evolution of an original idea that was good to begin with (CAT SL1, Merlin VSM, Atma M60s, Vandersteen 3A) for example, rather than product lines that change dramtically every 3-4 years, claiming new technlogy or circuit breakthroughs. The "evolvers" tend to have gotten it right the first time in the basic conception, and slowly over time there are real imporvments in passive parts that lead to refinement of the original, as opposed to a wholesale rethinking. I never though about that much, but the products that I own and have most enjoyed are coincetantlly those that sort themselves through time with minor improvemnts to the original.

It means you don't have the oldest version but a new version is forthcoming.