Perhaps something is amiss with your preamp. I have pretty much the same bass, from 1985. I also have the 5 string version from early 1991, right before the model makeover to different electronics and general body/neck changes.
--The 85 was great from the day I got it. Just needed the pots cleaned out and then set up to my tastes. I already loved Super Ferrites, but was also impressed with the preamp. It was rather advanced for a mid 80s design--it had all the things that bassists today want: a 3 band EQ, active/passive modes, adjustable active mode gain trim, a passive tone control and doesn't have a baked-in EQ curve in active mode.
--The 91 was a basket case. It needed a LOT of work, it had pots that didn't work, missing knobs, broken tuner, needed a new active/passive switch and the neck needed a trip to warpedneck.com. Finding replacement spider pots for this preamp is next to impossible, and finding a working replacement preamp is only
slightly less rare.
After a couple months of searching, I gave up and decided to roll up the sleeves. I took all of the pots off the board, took them apart, refurbished them and got a replacement switch. Luckily, it all worked and the rest of the preamp was functioning as it should.
The odd thing is the mid control, from what I understand, isn't technically 'active.' It's some sort of inductance circuit thing and doesn't require power from the battery. The bass and treble controls are proper active controls. I think this was to conserve the battery life. I do find that this preamp does eat batteries faster than any other active instrument I own. Perhaps something is off with this mid circuit? That could explain why it's not 'flat' sounding. I can assure you that active vs passive mode on these is pretty much the same tonality.
