The 81 is a good choice, as is the Invader. With Humbuckers, I reccommend keeping it around the 8K or higher ohms reading. Even a stock PAF is enough for Metal, that's what Tony Iommi, Edward Van-Halen, Steve Lukather, James Hetfield, Kirt Hammett, Dave Mustaine, and most of the 80's metal guys who started the whole metal high gain thing started off using.
The higher output the pickup, the warmer the sound, one problem I have with EMGs occasionally is they are a little too thick for tuning down, they make the sound a little too sloppy and muddy for my taste on certain amps, so then I usually jump to my Jaguar with it's moderate 10K ohm cool rail pickups for drop tuning stuff, or something with weaker humbuckers. The higher the output, the bassier the tone, unless the output is generated by the magnets, which seems to create a thinner higher frequency sound compared to higher amounts of windings.
