I've spent alot of time trying to find a compact preamp that runs on one 9V battery and has bass and treble centers at 40Hz and 4KHz, and is boost and cut as well (+/- 15 dB). My search led me to consider the Noll preamp
Noll preamps aren't available in the USA, but many top European luthiers install them (Ritter comes to mind). They also have parametric sytems (3 and 4 band) which are excellent.
Finally, I was able to order from a German vendor: www.BassParts.de
Rudiger Ziesemann is the contact person, and his website
made it easy (once you clicked the English version). I was
able to pay with Paypal, which converted Euro's to dollars
with no problem. He is extremely friendly and helpful
Price: very competitive, shipping of course is an extra, but when you consider the preamp comes wired and includes pots it's worth it.
I considered Audere (excellent, but careful measurement of my
tiny cavity made it a very tight squeeze), Aguilar OBP-2 (finally played one, the 6.5K treble center doesns't work with my finger style), and OBP-1 (boost only), great preamp, but two 9v batteries and inability to dial in a "cut treble" reggae style
Will post review when preamp is in the bass