Normally, if you want to "tighten" up the bass on a pickup, you want to reduce it, not increase it.
99% of the time, a tight pickup is a pickup with a low-end rolloff (like the Black Winter or the Nazgul), IME.
Personally, I've always felt the JB's "looseness" problem seems to be more of an issue with the low-mids being stuffy rather than the low-end being boomy. JME.