Oddly enough I have a KM-48 and I get mask marks with it when I use it on really cold water dives. But I also get mask marks with my other soft silicone skirted and frameless masks as well on 2 to 3 hour long dives.
At the other extreme, I never got mask marks with my EXO-26 on commercial dives and it has a comparatively hard skirt - but it has a lot of skirt area and as a true FFM, equalization occurs on every breathe so equalization could be part of it - but then again I wore the straps tighter because of the positive pressure aspects of the mask,so it's probably a wash
In my opinion, "mask face" is just the nature of the beast for a long technical dive. Equalizing the mask is not really the major issue, at least in my case, but I do tend to wear my masks comparatively tight to ensure they stay put in high flow caves, when having on a line in current and on scooter dives. I could probably avoid "mask face" by going very light on the strap, but then it would tend to dislodge when looking sideways in high flow, strong current or when scootering, and personally, I'd rather have mask face than have my mask periodically unseat and leak or flood. I can't see my mask face anyway, so it's someone else's problem if they are worried about it.
What I do find objectionable on a 2 or 3 hour long dive is having the strap too high on the back of my head. When that happens, after a couple hours the pressure under my nose starts to hurt, and once it starts it pretty much lasts the rest of the dive even if the strap is readjusted, so I take great care to position the strap low on my head when gearing up.
In short, I would not worry about "mask face" as a sign of improper mask strap tension, but rather focus on using just enough strap tension to get the job done. If you don't scooter, hang on a deco line in current, or swim in high flow caves, you can probably get by with a bit looser fit. But if you need a snugger fit, by all means do so.
