Okay here goes:
I had a total of +4 set affix (so do you on your set), I think this has become a standard for every top-tier set.
So now to the combination:
Druidic (5) gives +125% MP and MP Regeneration and also increases poison Dot dmg by (HP reg + MP reg)/4000 <- This is the important part.
Cosmic power 5 gives 125% MP and MP Reg and more dmg in timewarp, bit that part is not important
I also use blood magic but that doesnt make a difference as far as I can tell
Anyways, I have a crystal MP reg affix that gives 2764 MP reg and a HP reg affix that gives 656 HP reg. For simplicity lets assume that that equals a total of 3400 MP reg.
With that equip I ended up with 21.5k HP reg.
That equals a poison Dot damage increase of almost 550% with only 3400 MP reg on the equip. This should pretty much quintuple your DPS against targets that take little damage from crushing blow and I tested it with random gear!
Also, it is hilarious to watch your health bar go up after taking damage and you can spam your abilities without ever running out of mana.
EDIT: After some tweaking I have gotten to 27.5k HP reg by putting hero points in Intelligence instead of strength. That is approx 550 HP per second in arena. Its funny to watch some lower tier players attacking you and not being able to even get through your regen >.<