What is the best DD?

[wish12oz]

Don't believe you.

Regardless, Haste/Slow work the same way (50% haste + 25% slow = 25% haste), so it's not implausible that DT is additive instead of multiplicative. In fact I'm fairly sure it's additive.

I think its more like the effects of the spell slow and haste. Having haste (+damage) cast on you while you have slow(-damage) would overwrite and cancel out the slow.

Again I ask, can you prove how it works? Saying it's common sense is not proving that's how it works.