Me too, I never usually use the middle position (mix of neck & bridge) at all - it's normally either the neck humbucker or the bridge for me.
But with this set, I'll be using the middle a whole lot now.
Only way I can describe is it sounds a bit like the tone you get from a wah wah pedal, when you set it in position on the treble side. But better.
Separately, the bridge and neck pickups sound perfect - just as I'd expect them to, so I can't see how I've wired them out of phase or anything like that - but I'm open to suggestions if anyone knows what's going on here.