If your amp has a lot of high mids and you're looking for more low mid chunk, you're absolutely right, the JB is the wrong pickup. Especially in a neck though guitar like the King V. Neck through guitars that I've owned all tended to further accentuate the upper mids, and be little thin in the low end.
If you want to beef up the low mids, I second Benjy's suggestion of the PATB1. It's a very chunky sounding pickup, and it'll take some of the harsh edge off that you often get with guitars that have neck through construction and ebony fretboards.
Other pickups I'd recommend are the Custom, which has a fat, chunky lower midrange but plenty of high end sizzle too, or the Custom Custom, which has a mellower high end and fat chunky mids that crunch like a chainsaw. If your ebony fretbaord already makes the guitar sound edgy and bright, the Custom Custom might be the better choice.
The C5 may be good too. I agree with I M Ken, though the tone chart gives the impression it has a lot of midrange scooped out, the one I have in my Jackson PC-2 sounds as crunchy as any of the more midrangey pickups available. And there's a little more bottom, which also would probably complement a guitar with neck through construction.
BTW, I'm in no way knocking neck through guitars. I've owned several, still have a few, and I like them. But they do sound different than bolt on's or set necks, and their unique tonal properties are something you have to take into consideration if you're looking to install pickups to alter the tone. Have fun!
