To answer your question:
Q) John East Preamp
A) They are very good altho many think is pricy but i think it worth every penny, i installed the J-Retro on my 60's Classic jazz and it turn the bass to a monster. Compliment on tone all the time from people. It do colour the sound a bit like abit of bass boost by default but i think is a good colour.
Q) 18v or 9v?
A) i would recommend 9v, i can't see any benifit of wiring it to 18v ......
Q) Battery cover?
A) i'm not too sure about the M-Retro but for my J-Retro their are no battery cover and neither do i have to mod the bass for the cover. Is just a direct replace for the control plate with the battery clip inside the plate ........ So everytime you change the battery you have to unscrew the plate to change battery ....... Not too sure about M-Retro, i say you better email john east, he will get back very quick.