Took a peek at my stun DC during a run and it was at 74! No Tactics twisted in and no Spare Hand. Spare hand did pop it up to 79 - wai.
Since this wasn't normal I decided to test it out. I thought it might be related to the ED I was running.
And when I leveled the ED from 1-2 I think it added +2 to my SF DC.
I made a quick list of SF DCs in each ED. The ED levels are shown. I was running in Draconic when I noticed this.
Base +Spare Hand SF DCs in different EDs:
78 Draconic Incarnation (2)
56 Exalted Angel (1)
56 Fatesinger (3)
56 Fury of the Wild (4)
60 Grandmaster of Flowers (3)
62 Legendary Dreadnought (4)
72 Magister (3)
56 Shadowdancer (4)
56 Shiradi Champion (3)
56 Unyielding Sentinel (3)
Quick testing:
Spare hand adds +5
+2 wis ship buffs added +1 DC
Yugo added +1
Legendary Tactics twist didn't stack. Hmmm.
Couldn't figure out what that spell/song is that's cast on me in quests to increase my wis by +4 stacking. (Usually I'm hit with +4 str and +4 con and have to ask for the +4 wis.) Gotta get a pic so I can find it. Searching didn't work.
Once I figure it out, I'll ask someone to cast it on me to see if it stacks. Probably will fwiw.
I'm not seeing anything inherent in the EDs that supports this. When I get Draconic up to 3 I'll retest. I think each lvl in Draconic increases my SF DC by 2.
If you really really need pics, I can post them. Not bothering atm.