I have the 4 string '90 Jazz Plus. Bought it new in '91. One correction to your points above, there is a blend knob in the secondary ring under the volume knob, which controls the balance between the two p'ups. I too no longer use this bass with the battery, my snap cap broke and I've been playing it passive anyway almost exclusively, so did not see the need to spend any money getting that fixed. I find the tone very pleasing in passive mode, but if I was a gigging musician instead of a bedroom headliner, I would like the option to add the preamp when I wanted a wider range of tones.I own two 1990 Fender Jazz Plus V with Lace Sensors with the Kubicki preamp in it. This one pictured is my main squeeze and my first Fender. I don't mind the Lace Sensor pick ups but I pulled the battery because it doesn't sound to me like a Fender Jazz with the Kubicki pre on.
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This bass has been my primary and daily driver since I bought it new in 1990 from Guitar Center in Berkeley, CA (when there was a GC there). I was so heavily reliant on it in the 1990s that I bought another one of the same model and exact same year from Bass NW in the late 1990s. Her sister bass doesn't sound or play as well as the first bass shown here. Odd how that can be, hmm? </s>
It was made I think as some sort of knee-jerk marketing response to the new-at-that-time 5-string 'boutique bass kiddies' and only lasted a few years before they went back to the standard controls on the chrome plate. I heard that they stopped making them because of a problem between Kubicki & Fender but I can't imagine that they sold many of these either. They weren't the Jazz Bases that the kiddies were looking for so they didn't buy them. Despite all of that I still love this well made instrument.
The boost setting isn't much of a boost, but it can be adjusted via a pot accessible in the control cavity. As it is, I think it give an 8db boost to the mids.
One other point about "why" they made this bass - - it has a smaller body than the standard J and is slightly lighter, has a different feel to it. Plus of course the Kubicki preamp. I think the smaller body and slightly more neck heavy balance was more of a reason it wasn't popular, plus, it doesn't look like a "standard" J bass. Here are some pics of mine:
