I mean the buffer in the split n blend is probably causing the issue. You could add a pickup simulator between the Mammoth and blend buffer to mitigate this, though it will still change the sound somewhat compared to just the fuzz with no blend.
http://www.muzique.com/lab/pickups.htmWhen I was making a clean blend circuit that would work with an FF for Zach I found it was ultimately necessary to include a pickup simulator to get the fuzz to work properly.
I used the simpler one with the transformer