Testing out damage taken by high level monsters, naked and with gear on,
level 42 pug versus level 93 gob
without clothes def 25 4898
with clothes def 302, 2641

this suggests that dlevels effect multiplies the effects of your gear, if this is the case, it means your gear is always important.

probably dlevel is a curve, that multiplies the damage you take or give by a certain value, it operates to have a greater effect the larger the level difference in an exponential manner, it might be it may be calculated after everything else. The key here is it means your gear should still be effective at all levels, if it is not, it is most likely that the monsters stats/gear/traits versus your stats/gear/traits are the issue.