You can buy the SB preamp from Carvin.
I actually purchased the preamp and ran it in my Marcus Miller Jazz. I recently sold the body from that, purchased a warmoth p-bass body, put the marcus miller neck on it, and put the carvin preamp in that. I really like this preamp, and the price on them is pretty decent. The only real downside to it is that it is 18v and it can be a pain to find a place to tuck two 9v batteries in a bass that wasn't made for it.
I can see someone not wanting to spend a huge amount to rout a squire, drop in a couple battery boxes, and wire up a carvin preamp.
