I take mine down to the 10-14 psi range. As you stated, lower pressure = increased traction. The lower you go, the more chance you have of loosing a bead, though (I've only ever lost one; and it was after cutting a sidewall... so the tire was effectively at 0 psi). A lot of how low you can go has to do with the width of the tire compared to the width of the wheel. A skinnier wheel will help with bead retention versus a wider wheel with the same tire... but you also don't want the wheel too skinny for a DD because it will make the tires crown and cause uneven wear. My OEM's were 31x10.50 on 15x7 wheels and I now run 32x11.50 on 15x8's. The 32x11.50's on my OEM 15x7's crowned a bit much for my liking. Anyway, after a bit of explaning the method behind the madness, I'd say take them down to around 18psi next time and see how you like it. If you are worried about loosing a bead, I wouldn't consider it an issue at that pressure.