Late in on this, but just my experience:
running at 831>sppre>jb3 and also CMC8>sppre>jb3 was NOT a success - if it was set at 29, it brickwalled (badly) and at 0, I couldn't get the gain up enough to get a decent signal even if I boosted it on the JB3 itself. I was disappointed, since the budget wouldn't stretch to an AD20 with the mod for 1/8 and plug in power. (I'm recording classical singing - all acoustic)
The budget-friendly solution for me was a Reactive preamp (available at
www.reactivesounds.com or
www.minidisco.com); similar in spec to the SP version, but for whatever reason I can easily get the right levels w/both sets of mics - it's a simple knob affair and you just turn it till you see the clipping light thhen knock it back a degree or two. No, it's not in the league of the Denecke and its friends, but it does the job, w/the JB3 internal AD taking care of that side of things. I can get good levels, it was w/in my budget and it's doing adequately until I can afford something significantly better.
FWIW.