I also use 3 Marumi NDs and a Marumi Macro filter. But clever me I got 52mm thread cause I didn't think I'd need more than that cause it covers my 20mm and 50mm. But then I got Samyang 35mm which has 77mm. LOL
I'll be doing sharpness and color shift testing this weekend for a variety of focal lengths and will post my results.
Also have a linear polarizer from Marumi on backorder for general use. Stacking two would make a DIY variable option, but I'm worried about color shifts. http://www.2filter.com/marumi/marumicirpolarizers.html (bottom of page)
The Tiffen variable ND looks promising, but supposedly there was a production issue in the first batch. Much cheaper than Singh Ray, but still not nearly as cheap as a DIY option.
I'd like to buy some polarizers(linear and/or circular) and have started to do some research on how to lessen the color shift of DIY Variable-ND ( or any Vari-ND for that matter)
"This carefully matched set of filters work in combination to produce a density range from 1-1/3 stop loss to 11 stops of light loss. By selecting the best quality materials, Schneider variable ND offer 11stops (ND3.3) attenuation with no blue color shift at maximum density which is a common problem with inferior and unmatched materials common in lesser variable ND filters."
Finding a couple Old, Used, High Quality, Linear Polarizers will better fit my budget.