lvs

An attenuation of around 4dB with the mini pot at maximum. But it will kill brightness and even have the effect of a lowpass tone control. Bridging the inserted resistance with a cap of around 680picoF will restore resonance though; it may even restore too much brightness, and ask for an extra resistor in series with the cap itself. A sideeffect of the cap will also be a slighty different tone when the two pickups work together in parallel. Maybe you could do a simple test with a cap and 220K trimpot first, and if you think it has potential, insert a 20K trimpot in series with the cap for fine tweaking if needed. Finally, it would be best to replace the trims with fixed resistors. A few caps between 470-680p together with a trimpot (or two) don't cost much money.

From iraggie's last question I understood he wanted two volumes in parallel as well as in series. I wanted to point out that it's possible but also that is has a drawback.

lovekraft, oops mate, I reacted a bit too fast I see. Here's what I typed before your edit : I respectfully disagree. In my Strat I use one pot as a fader for the neck pickup in series with the bridge pickup at position 5 of the selector. It's a 250K log pot (previously tone control) shunting the neck pickup as a variable resistor. It works fine, I use it all the time. I also made a 2HB diagram for someone that works how I described in my previous post (only difference is no out of phase switch) and got some good feedback.

For your last question there exists a solution with two audio taper volume pots and a DPDT switch, making a push-pull pot or a separate toggle switch both possible. Switch position 1 gives pickups in parallel, each one connected to it's corresponding volume pot working in the traditional Jazz Bass way, ie independent from the other volume pot. Nothing new. Switch position 2 gives pickups in series, each one shunted with it's corresponding volume pot now working as a variable resistor. Though technically possible, a drawback however is that in the series combination, pots don't work ideal : starting from 0, volume increase is smooth, but already close to maximum at about 60% rotation with 250K pots. Pots of lower resistance could improve that, but would reduce pickup clarity and maximum volume especially in parallel. So your volume controls will respond differently in series vs. parallel. Wether that's acceptable, I think that decision is yours. If you want I'll send you a diagram.

...me too, and I would wire it up so that the volume pots work independent (especially to make parallel out of phase tweakable) when in parallel, and that one pot is master volume and the other can fade in one pickup when in series.

If I understood well, you would only use the middle pickup to combine for a Strat #4 sound. Since you're obviously not afraid of some experimenting, you might want to try a mod that J. Donahue uses in his Tele, making a Strat inbetween sound possible without having a pickup in the middle. It goes like this : a coil at the bridge is switched in parallel with one at the neck. Normally that gives the inbetween tone known from Telecaster. But, reversing one coil's connections, you get a parallel out of phase sound. Then inserting a (approximately) 10nanoF capacitor in series with one of the coils will shift relative phase, eliminating the boinginess and bring it into Stratocaster inbetween area, without using a middle pickup. I tried this with a 2SC guitar, but never with splitted HB coils. As you probably know, tricks like this are often close but no cigar (and then you have to integrate it into your guitar's switching system), but on the other hand it's no extra investment and not a awful lot of work to try it.