Reloaded the quick save, ran basically the same tests again. Target is a Level 11 Arcane Horror at Victory Rise. This is for the current version of the game, no SP story DLC just Collector Edition stuff and Black Emporium.
Setup 1 - Magic Defense Helm


Magic Defense 46
Spirit Resist 30
Damage 42-46
Setup 2 - Magic Defense Helm, Spirit Resist Tonic


Magic Defense 46
Spirit Resist 80
Damage 12-13
Setup 3 - Electric Helm


Magic Defense 33
Spirit Resist 30
Damage 53-57
Setup 4 - Electric Helm, Spirit Resist Tonic


Magic Defense 33
Spirit Resist 80
Damage 15-16
Setup 5 - Spirit Helm


Magic Defense 33
Spirit Resist 55
Damage 34-37
Setup 6 - Spirit Helm, Spirit Resist Tonic


Magic Defense 33
Spirit Resist 80
Damage 15-16
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In the mechanics thread, the interaction between defense and resistances is shown as multiplicative in a manner similar to this (ignoring other modifiers which should be constant):
Damage_Taken = Base_Damage*(1 - Defense)*(1 - Resistance)
Assuming this is working correctly then the above setups would have been taking the following fraction of base damage per hit:
Magic Defense Helm: 0.378
Magic Defense Helm + Spirit Resist Tonic: 0.108
Electric Helm: 0.469
Electric Helm + Spirit Resist Tonic: 0.134
Spirit Helm: 0.3015
Spirit Helm + Spirit Resist Tonic: 0.134