No worries. It's a good question.
The OBP-3 is like the USA 18v circuit. It is both JFET and op-amp based. It is also 3-band.
Unlike the USA 9v circuit or the TonePump, the OBP-3 is a much cleaner circuit offering a little coloration of the tone, so you get a more accurate expression of the wood combinations working with the pups and circuit.
The TonePump is much more gritty and growly. And is supposed to sound like the USA 9v.
Since bolt-on models typically have a noticeable difference in tonal profile from neck-thru designs, Stuart told me that he wanted to USA Bolt-On models to offer a different range of tone than the neck-thru models.
This was for two reasons. First, the wood (mahogany and a top of maple or some other tone-wood) and the frequency and sustain differences inherent in the bolt-on design would make it more difficult to sound identical to the NS-2 anyway. So why try?
It also meant that there's another tonal range, more traditional sounding in tone but still offering some extra sustain (with the deep pocket neck joint) and the high quality of the woods used that he could offer musicians who may not want an aggressive bass.
To many of us the beauty of the NS is the body, the tone is a bonus. To Stuart, it's about the tone. But he saw that offering the NS body with a more traditional bolt-on design and sound would be appealing to a lot of bassists who wouldn't look twice at the neck thru NS.
I think the decision was to broaden the appeal of Spector basses by offering something with that hand-made quality and a great tone, but making it different enough to not sound JUST like the neck-thru models. I mean, if you like that tone then why not get a neck-thru?
Lastly, Aguilar has been very responsive to Spector regarding the refinement of their circuits and more recently, their pickups (used in the Coda). I've actually done some photography work for Aguilar and I use their amps. They are good people with the same passion for making the best quality equipment possible that Stuart and PJ share.
Plus like Stuart and PJ, Dave and Dave (Boonshoft and Avenius) are both musicians and they love other musicians.
TonePumps can be put into any USA if you request. Like Rex Brown. He prefers the TonePump over the USA circuits. He's mainly playing Euro4LX models right now, but has an RX-T or two and usually plays one or two songs during Kill Devil Hill's sets with them. But they all have the same circuit, the TonePump.
He grabbed the white Euro4LX-Limited Edition model off the wall at NAMM and played it... walked it over to me (PJ was talking to someone) and said, "I want this. This one. This one right here... it's perfect." I said, "No worries." and he handed me the bass. And when PJ was done, I walked it over (Rex had gone over and was talking with Stuart) and I said to PJ, "Rex wants this." And PJ laughed and said, "OK!"
The RX-T has the TonePump in it stock. That's a USA model with the TP in it.
With Stuart the purpose of just about everything he does has to do with tone. He's fanatical about the sound of anything he makes.
I hope this answers your question!