Odd transformer behavior

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I have a 1:1 transformer wired up as a phase switch: the secondary is simply reversed via switch to invert absolute phase. However, one setting is dramatically better, seemingly outside the realm of absolute phase differences- dead treble, etc. And I mean dead.

Any ideas? It's wired correctly and in place between my linestage and active XO. The only thing I can think of is some goofball ground shunting.
 
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