I find airing down to about 20 psi/138 kPa front, 25psi/172kPa rear works about as good as it gets with my Dunlop Trail Max's in sand/thick gravel. They are better than the stock Bridgestones, but still squirrely. Knobbies would be better.I would add that getting as far back as you can on soft sand is very important on the strom...
Makes soft sand ssoooo much easier..
The 650 Strom is THE worst bike I've ever ridden in sand.
Very short travel, harsh suspension like the Stroms, is unforgiving in sand.
Also, on wet days, I wind out most of the preload front and rear. This really helps in mud, especially that greasy red clay we get around here...
If I was to ride a lot of sand & mud, I'd get bar risers and setbacks to stand more upright. I feel like I am too leaned forward standing with the stock setup.
US-spec DL650s do not have adjustable front preload.