TBX tone control on my 3 PJ basses. I always loved the PJ configuration with the standard tone fully clockwise for max treble. Great sound at that one setting, but that was it. Not a whole lot of range, I found, regardless if V/V/T or V/B/T or switch vs. no switch. Then I tried a 90's MIJ PJ bass (what the Duff McKagan model is based on essentially) and was really impressed how the tone was much more dynamic and "open" without as much volume drop with both pickups. Thought it might be the Japanese pickups, then discovered the secret was with the TBX tone control. The reduced resistance allows the brightness of the J pickup to shine through and both pickups in tandem have better punch.
"The Treble Bass Expander (TBX) is a stacked and detented 250K/1Meg potentiometer that expands your tonal palette. It functions as a standard tone control from 1 to 5, after which it decreases resistance, sending more bass, treble, presence and output to your amp."
No longer have that MIJ PJ ironically, but the three PJs I now have each has a TBX control. Would highly recommend for any PJ ... not sure why it wouldn't be stock on all Fender PJ's (is not a pricey part to add).