The degrees of strength between filters in a series is not all that scientific, it's sort of an eyeball thing on the part of the filter designer, so it is very hard to compare strengths between different filters, especially a 1960's (or earlier) filter like a Tiffen Low-Con to an 1980's filter like a Tiffen Black ProMist.
The differences between the old pre-1970's LowCons, Fogs, and Double Fogs was mainly in the mist particles that caused halation -- the Low-Cons use a much finer grade of particles so that the contrast loss is spread out more through the filter rather than localized around a light source, though a Low-Con is similar to a Fog, just that Fogs are, well, foggier. "Bound for Glory", for example, used Low-Cons and Fogs in different scenes, depending on how much glowing around lights Haskell Wexler wanted to get.
Double Fogs are not actually twice as heavy as Fogs, but are a cross between a Low-Con and a Fog.
Then after 1980, Tiffen made a filter where the particles were so fine that the haze created was so even that there was no localized glow around lights -- they called these Ultra-Cons.
Now Schneider makes a Low-Con but it may be closer to Tiffen's Ultra-Con, I'm not sure.
The older design of Tiffen's Low-Cons and Fogs has some minor problems, there aren't enough clear areas between the particles for a sharp image to pass through so the filters do cause a little loss of sharpness, some tiny amount of blurriness. More so with Fogs. ProMists, from the 1980's, seemed to solve that particular problem though they are designed to soften detail as well as create halation around lights, but you don't get that blurred quality. But it's pretty subtle with those old filters and some of the loss of sharpness is just due to loss of contrast.
If what you are after is an "organic" quality, well, that's fairly subjective and is something you'll have to test and judge yourself -- no one designs a filter to be more "organic".