Never occured to me, but reading the lines I would say that the magnitude is substarcted from the stat that is used in the save roll.
The used stat is the one given in the Save-line of the power manifested, e.g. for a Cardinals Crimson Blow even from AR.
Sounds a bit weird to me, but following the lines of the rules it seems to come out this way.
BUT that would mean that an enhanced Cardinals Crimson Blow would have an effective DMG of 17 (!) if doing it with a magnitude of 6 [11 for the DMG and another -6 from the magnitude]

For this one I would present this again for the FAQ-team...
And for the PW-satat for non-casters:
Iīd say itīs a stat that simply all models have. Possible it comes from earlier editions where the Resist-rolls were made against PW-stats.
Further it would be a bit unlogical to give the PW stat (a stat for ability/strentgh of mind) only to channelers. All have a mind and they have all astst line, even if its useless for many of them.....
And last but not least there is the power "Power Void". Itīs Save is against PW. It can be cast against channelers, but has a template effect.
Possible that other affected models are caught under it.
I would give them save against PW, where their PW comes in. And if they miss it they would have to roll the LD tests for performing anctions as noted in the spell.
But these are guesses.
Just put it up on the FAQ section.