Here's what I got from Bruce today, regarding the pickups and controls:
Here's a quick overview:
The M-pickup is the cluster of 4 coils in the center of the body. They're made to give a thick, warm electric bass sound. You can adjust them individually in height with a 3/32" Allen wrench, to change the balance in levels between strings. Each coil has three 1/4" Alnico V magnets, so they're quite powerful.
The P-pickup is down inside the body, under the bridge. This is a mechanical/percussive pickup unlike anything on any other bass. The bridge is mounted on a suspension system of tuned spring bars, so it physically twitches and moves when you pluck the strings. The motion is similar to how the bridge moves on an upright bass, and it adds a pop and warble to the attack profile, and some sympathetic harmonics across the strings. The effect is very dynamic; the harder you pluck, the more you hear. Two magnets hang down from the suspension frame into a pair of coils mounted deep down in the body, which capture the movement and turn it into signal.
The output jack closest to the knobs is the "normal" output, which you'll use most of the time. The Volume, Tone, and Blend controls only work through the Normal output jack.
The knob at the bottom is the Tone control. It's a passive high roll-off, with a progressive rate. Fully clockwise is wide open; counter-clockwise cuts off most of the high end above 1khz.
The knob nearest the tailpiece is the Blend control, which is a linear blend between the M-pickup and the P-pickup. Fully clockwise is all P-pickup; fully counter-clockwise is all M-pickup.
The two output jacks up against the strings are the Bypass jacks. The one nearest the bridge is for the M-pickup, and the one nearest the tailpiece is for the P-pickup. When you plug into either of the Bypass jacks, it disconnects that pickup from the rest of the circuit and connects it directly to the Bypass jack. You'll find that the sound directly from each pickup is clearer and has more frequency range than it does going through the control circuit. In the studio, you can plug two cords into the Bypass jacks, running into two channels in the recording console. This will allow you to do one take, and have two separate tracks of the two pickups, fully isolated and synched together. You can them blend them as you like during mixdown.
I also use the M-pickup Bypass Jack (the one closest to the bridge) to quickly plug in my electronic tuner. The thick, warbling tone of the P-pickup can confuse some electronic tuners.
A quick note about your bass: While #066 was here, I did some experimenting with its P-pickup, trying some new ideas and comparing the sound to my own #060. I learned a few things and ended up incorporating two design changes in #066 that worked very well. They will now become standard, and I may retrofit #060 and others. One change was switching to larger 3/8" x 1" Alnico 5 magnets, in place of the previous 1/4" x 1" magnets. This boosted the output level of the P-pickup some, but mostly it boosted the very low subharmonics. The second change involves the shape of two rubber damping blocks that reduce two harmonic points in the spring bars. Without them, the P-pickup would have a slight annoying ring at two spots on the fingerboard. Previously, I had been using rectangular rubber blocks that clamped around the spring bars which worked okay. However, experimenting on #066, I found that small wedge-shaped rubber blocks worked even better. They damped the harmonics, with out as much overall damping of the pickup's tone. The R & D never ends here!
Other trivia: #066 is the first fretted Series IV model that I've built. I spent some time playing it, and even took it (along with #060) to one of my bluegrass jam sessions. I photographed it well, and it will be shown prominently in my upgraded web site....whenever I get it finished!
