I never seen an auto attack build with Powerstaff before, but as far as we know about magic amplification. It adds max magic damage range onto your original magic damage.
So magic amplification effect both spell and auto attack (Only if the auto attack is scaled off magic attack from using Powerstaff).
Additionally, Powerstaffs doesn’t make your attack purely magic, but instead, it adding magic attack property onto your based damage property itself. Think of sacrament as an example.
Meaning, you still require some DEX or any form of accuracy to land your attacks as well.
To be more accurate. Here is similar statement from a person name Evochron13
Lethargy: Additional damage affects all “Strike” classified damage so if you’re using the melee attack (default C) for Power Staff yes it would effect it.
Thaumaturge’s Swell Left Arm increases both attack and magic attack with Power Staff you would get double dipping from it.
The staff states “Staff attacks (C Key) increases damage equal to 200% of your magic attack” in crazy backwards translation land, it means your auto attacks physical attack gets additional damage equal to 200% of your MATK. HOWEVER, your staff attack is a melee attack and susceptible to block and dodge. Meaning you’ll need some form of accuracy or DEX to do so. I am not 100% sure how accurate you will be with base DEX and base SPR levels.
Personally I plan to be running/testing a Wiz1Pyro3Thauma3 focusing on auto attacks and stat pumping CON so that Thauma 3’s Swell Brain and then Transpose will give me INT = CON and CON = 149 which is what Swell Brain’s INT at capped attribute and rank would be. This would be a non powerstaff based build.