I have owned my Spector rebop 4 for about six years. I love the way it plays, but have seldom used it live or at practice because of the Tone Pump preamp (old one with no trim pot) does not play well with some of my amps and I did not like the hollow scooped sound it produced. I found myself constantly tweaking the bass and treble knobs trying to dial out the muddy sound.
So recently I started looking at replacement preamps - until I read on the Spector website that Mike Kroeger's MK5 bass had EMG HZ and a passive tone stack and this got me thinking - what does the HZ's sound like passive? So I started digging and found an old TB post from "Johny810" about how to bypass the Tone Pump without removing the preamp and ripping out and replacing it with passive tone pots. "If you remove the battery and put a jumper from the white lead on one of the volume pots to the purple wire on the output jack - this will make the circuit passive."
So I gave it a shot-----and----- I really dig the passive tone. I think I will run it like this for a couple of months before I make a final decision, but for now I am thinking that I may not put an active preamp in the bass.
Yes, I lost some volume but it sounds like the Rebop now has some mids and I am not overdriving the front end of my amp. Also it cleared up some of the boomy low-end the Tone Pump always seemed to infuse in the tone - unless the bass pot was dialed all the way back (-4db @ 55hz).
Just thought I would share my experience.
Update 10/13/13: decided I missed the TonePump due to the lack of output of the passive only circut and the dull lifeless top-end. Ordered the newer TonePump with the adj trim pot.