passive Fender into active and vice versa

Hi!
I've got 1977 classical Fender Jazz bass. Sounds great. I started playing in a big band, where I would use some mid-song knob tweaking - get louder for a while, more mids etc. Floorboard does not solve it, I need to do it according to what I hear, not according to what I set in advance. Active electronics would solve it perfectly. But, I don't want to cut new holes into the bass, feels barbarish to me. Is there any active electronics circuitry on the market, that would easily fit into classical jazz bass body?
Thanks a lot!

 

Thanks for tips! I went for Audere JZ, on the sole reason that there were necessary dimensions of the cavity written in the manual, so I could check that it would fit before I ordered.
Yesterday I mounted it. It was easy enough, though I had to resolder grounding cables on the cavity sheet because there was a big blop of tin right under the battery.

I upload music samples for anybody to check the difference. Recorded straight to soundcard, no afterwork, audere inside caps and pots unchanged, all the tone knobs in middle position:

jazzbass-passive original.mp3
both pickups - bridge - neck - both slap - neck slap

jazzbass-audere pickups.mp3
both - bridge - neck - both slap - neck slap

jazzbass-audere Zbutton.mp3
both pickups. Z button middle - hi - lo

The first two samples show that compared to original passive, there's a significant gain increase, as well as widening of the sound spectrum and dynamic range. At first I was surprised by the change of overall sound character - the bass has a different feel now. I imagined that the character of the bass is determined by its strenghts as well as its imperfections. But in a few minutes I got used to it and now I feel I can play the same sound as before, and many (many!) more.

The Z-Switch in its default setup gives too brutal tonal corrections in my opinion; I'll try some tweaking but I don't expect there will really be a need to use it.

The tone corrections are sensitive, a little move makes the difference I want. Connected directly to link, I'd prefer the highs and hi mids to be placed a bit higher, the lower corrections seem right. Experience tells me that on a gig it would be perfectly effective.

 

After almost a year, I add to my Audere review:
- the battery lasts forever (haven't changed so far... but I practise mostly on upright, I use the bass only like two evenings in a week)
- the Z button, after some tweaking, is useful; in addition to the basic sound, I found a good setup for 'motown sound' and 'slap sound', both of these I use rarely, but come in handy anyway
- in my first impressions, I had a feeling that the lo-mid and hi-mid frequencies should have been higher; I reconsidered, I use them with no complaints.
- from technical standpoint, the bass now sound right with almost any rig, directly to mix, or with any cable I use. This is because active impedance matching and great EQ options. Makes it much more practical for me... I didn't have 'stage problems' since I installed it.