Yes, I would really like to know if they are stacked additively or multiplicitavely.

If I had to guess, I would hazard that they are stacked additively, but I really don't know.

it would add the % together , so if u had 2 helms with +100% it would be like having one with +200%, if i were to guess, but some clearafication would be good

To my testing, this is indeed how it works, it adds your % bonuses together before applying them to your static +#/sec.

you'll have to forgive me for a memory lapse, else i would surely give credit where its due and link the thread.

however...i have a distinct recollection of seeing some number crunching someone did and they came up with the conclusion that percent bonuses stack additively with one another. so if you have a +10% item and a +25% item you are gaining +35% to that stat. the same thread author (really wish i could remember who it was, sorry) also determined that percent bonuses are applied LAST meaning they increase all of your constant bonuses as well as your base stats. 

Thank you very much for that very informative reply.

For the critical stuff on the other hand it's different

Each item will keep its effect and % but you can possibly have all of them together

For exemple the gauntlets Critical x2 and Ashkandor Critical x4 and the Assassin's Plate Critical x1.5

If they all trigger you'll have 4x2x1.5 = x12 Critical Hit. Chances are low, but stacking up critical gear is a good idea:)