I tried every combination possible when I added the middle pickup and tried using both a toaster and a high-gain there. In short, I didn't think it really added much to the overall sound. I guess I was expecting that mixing it's signal in with either the neck or bridge circuits would give a more powerful or fuller tone than just the normal 2-PU system provided. That really wasn't the case, so most of the time the mid pickup was turned off. I even bought a Ric-O-Sound box and fooled with various versions of stereo and it still didn't thrill me like I'd hoped it would.
I finally settled on a rather curious system. The neck toaster and bridge high-gain are wired to the pots, the toggle and fifth knob, just like they would be on a normal 360. Their output is run to a normal mono output jack on the jack plate . The middle toaster is wired all by itself. It has it's own volume knob (suggested by Dale and a great idea) but no tone pot - it's tone is essentially turned all the way up, all of the time. I pulled the stereo output jack out of the jack plate and installed a second mono jack. The middle pickup is wired to this jack all by itself. I use two cords and run into two amps or two channels of the same amp. I could have run it all through one stereo cord and matching jack and used a splitter at the amp, but I ran my main bass with two cords for years. It's simple and I'm used to it, so I stuck with it.
To start with, the middle toaster run hot and straight into the amp has beautiful tone, even if that's all your using. The other cord essentially generates the normal sounds of a 360-12. I can mix the two signals and adjust their individual tones as I desire, and also run one cord through the JangleBox or other electronic tone-shaper and blend in as much as I want into the overall mix. I love the JangleBox, but was never really happy with the way it seemed to lose the low end of the sound. Now I just boost the low end by mixing in the other (non-Jangled) channel. I can also record if I want by running one cord directly into the recorder and using the built-in effects and running my other cord to my amp (which seems to have nicer tone than the Korg recorder) and sticking a microphone in front of the amp. Record on two channels and you can sit and tweak the overall sound in any direction you want.
Yes, it's an unusual mod, but I now feel like my middle pickup is actually doing something. When you kick it on, especially when you're just playing and it's run through a second amp or second channel with different tone settings, it really generates in a much fuller sound. I may be the Dr. Frankenstein of Ricervania, but compared to my other Ric twelves, this sucker really kicks some serious butt.
The second pickup installation is not for the faint-hearted. You'll most likely need to dig a trench for the underside of the pickup, right in the middle of a perfectly good Rickenbacker. Rather than disassemble the entire guitar to be able to get a router in there, I used the old method of a series of overlapping depressions (maybe 3/16"-1/4" deep) drilled with big forstner bit on a hand drill. Then I drilled the small diagonal tunnel connecting the trench to the control cavity and drilled the pickup screw holes. I sealed the fresh "rout" with varnish and trimmed the guards to fit the new pickup, like those on a 370 (I eventually made new back-painted guards to match the TRC that came with the guitar). For somebody who is a decent woodworker but not a luthier, the mid pup installation is do-able, but a pretty scary job. I would do it again in a heartbeat to get my stereo sound, but If all I was doing was trying to turn a 360 into a stock 370, I don't think it adds enough sound to be worth the risk, expense and anything else that might go along with having a modded guitar.
The sixth knob is the middle pickup volume pot and the bridge pickup is a high-gain converted to take a toaster cover.
