If you follow one piece of advice I give follow this: don't by hz's.
I have a set of em in my fretless but they are the most dull sounding lame crap I've ever heard... maybe when I put a pre in there it will sound a bit better, but for now my opinion is they blow.
I've had bartolinis... I've had dimarzios, I've had basslines... I've had fenders... all of which are good.
Honestly my suggestion would be to get a good set of stacked single j pickups... then get a bartolini preamp.
I've tried a few things in my p-bass and all sounded good but I settled on an alnico (ernie ball style) humbucker and a dimarzio split p going into a bartolini pre.
another mod: screw the blend pot on the jbass... I'd go with a three way toggle. blends are just lame IMO cause one pickup is always overpowering the other... unlike the middle position on a toggle.
Unfortunately, to get the preamps bass and treble controls, plus a volume, plus a toggle... you'd nead a stacked pot... or you could drill one small extra hole and go with a mini on/on/on (for the three way) but you'd have to wire it so that both pickups are always on and when you flip it one way or the other you short out the p/u... I could draw you a diagram if you want to go this route... lmk.
I'd also wire in a bypass push/pull for the preamp so you can go to straight passives when you want to.
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