However, everyone feel free to continue to comment on this if you are so inspired!
Here are my thoughts...
First, with the 70's pickup placement, yeah, the bridge pickup can be a bit thin, BUT, have you tried just favoring it enough to get the edge/cut you want, and then backing the VTC off 30-50%, AND boosting the bass? That will give you more mids both in the upper range (from the bridge favoring) and the lower mids (from the bass control both filling in space and also reaching somewhat into the low mids just from the nature of how it works). I've had pretty good luck with that.
That said, for the reasons you state, that's one of the reasons I really dig the 60's placement the most these days. I used to be ALL about the 70's spec, but more lately have really gotten into the 60's spec more.
Ok, so let's say that doesn't do it for you...
The WL preamp isn't an option, since you can't buy that aftermarket, so toss that out.
Other preamps? Man, I hate to go away from the magic of the Sadowsky preamp, but if it really isn't quite doing it for you AND you can't afford, or don't wish to consider a different model, then I'd suggest you get into a preamp that either has sweepable mids (like a J-Retro) or something that has either internally settable mid freqs that include something down around 250-400Hz or where the preamp's mids are otherwise somehow configurable to that frequency range. I could be off-base here, but I'm getting the feeling that's the zone you're missing with the 70's spec. At least that's the area of the mids I always felt sounded the best boosting on a 70's spec bass.
Ok, so what about other models of basses? Assuming you're primarily a J guy, I'd definitely suggest getting one of Roger's more typical 60's spec basses. That's all I own and I NEVER EVER have a problem with a fat cutting tone. NEVER. Otherwise, nostatic is absolutley correct, if you really want low mid grunt and you don't mind losing more of the classic J tone for something a bit more generally "modern" sounding (sorta'), the PJ is a great solution. That P pickup imparts a great earthy mid character to the sound, and even if you're not a P guy, try soloing that pickup sometime. I'm pretty sure you'll be pleasantly surprised at how well it "works".
Anyway, there's my array of thoughts, depending on your goals and pocketbook, and my first recommendation is to try and make your bass work for you without mod'ing it, but that's for you to decide. Hope it's of some help.