Wiring for a "passive / 9V / 18V" all-in-one switch?

[Vicar in a Tutu]

Here's a tricky conundrum for some of you clever modders to get your head around. I have an Aerodyne Jazz bass (100% passive) that I will be replacing ALL electronics from as soon as I've got my circuit figured out, and when I can therefore order the required components.

[u]Shopping list so far includes:[/u]
Seymour Duncan Quarter Pound P/J Bass Set
Seymour Duncan STC-2P Preamp
Gotoh 18V Double Battery Box
[b]??? mystery switch ???[/b]

I will need to rout for the preamp and battery box, and drill through for the switch.


[b]The aim:[/b]
I want to install a 3-way switch (preferably a round rocker in the style of the one pictured below) that will allow me to select:

[b]9V Active[/b] (2 batteries in parallel)
[b]Passive[/b] (bypass preamp + batteries)
[b]18V Active[/b] (2 batteries in series)



This is an easy project if I use two switches as in the diagram. However, I am going for simplicity and aesthetics, surely it must be possible with only one? I assume I will need at least an 8 pin on/on/on, can anybody help with sourcing this elusive switch and a wiring diagram?

[color=#141414][font=verdana, geneva, lucida,]Here is the preamp circuit for reference:[/font][/color]
[color=#141414][font=verdana, geneva, lucida,][/font][/color]

Thanks in advance.