I’m not familiar with the Magnepan setup, or the resistors they provide, but many wirewound resistors have an unwanted inductance byproduct due to how they’re wound. You should be better off if you can find non-inductive resistors of the same value...many of those are double wound in a manner that cancels the inductive byproduct.
The ones I'm currently using in my speakers for tweeter attenuation are old Dale metal body 25W 3 ohm designated with an "NH" prefix.
https://forum.digikey.com/t/inductive-vs-non-inductive-resistors/62