Actually... because the USA 9v is made by hand... the cost and time associated with making the circuits wasn't logical for a bass that Spector planned on making at least twice as many on a monthly basis.
Also, the JFET tech is really much better. There is no signal loss from the pickups through the circuit due to oversaturation during the note's attack.
PJ and Stuart saw it as a way to get the newer tech into Spector basses and improve the overall performance of the circuit.
The ONLY op-amp based circuit Spector uses anymore is the USA 9v. And that circuit is so growly because Henry (at Haz) and Stuart designed an array of op-amps to handle the over-sat issue, but this distorts the tone (as in overdrive type distortion).
PJ flew over to the Czech Republic with his famous Brooklyn NS-2 #603 (you can see this bass here next to PJ's biography:
Link Removed ) and sat side-by-side with the circuit's designer to get the tone-color of the TonePump as close to his NS-2 as possible.
So regarding the TonePump, PJ's main bass (which I know he plays more than any other that he owns... I've played it and it's AMAZING!) is the master "template" for the TonePump.
Some history that that's not on any of the official sites for you TonePump lovers out there.