I like an underwound single in the middle. I've got some in the 5's. With humbuckers, it's a crapshoot. Just saying you want an underwound humbucker won't necessarily yield the same result as the single coil recipe, because you've got different magnet types to consider, and in a series humbucker setup, one coil sort of "feeds" the other coil. Going too weak can start to underpower the series link so to speak. I've got a 7.4k Alnico 4 bridge humbucker in one Mahogany guitar with that underwound middle single in the middle. Overall it's a very sweet guitar with a great tonality to it. But under mid-to-heavy overdrive you can definitely tell that the notes aren't jumping out of the bridge humbucker. It doesn't saturate preamps the same way. I would deliberately have to raise the gain knob or use a booster to achieve the same levels of saturation, even vs. a 59B for example. But the tone is worth it on that particular guitar. I won't swap the bridge pickup. I like it. Maybe I'll put a clean boost on the guitar pot someday, but the other two pickups are true single coils, so then I'll be boosting the noise before I hit the amp, so my cleans might be noisier than necessary.
Anyway, don't fear the underwound bucker, but know that it's really almost a different animal than PAF or higher winds.
