A passive pickup is brightest when wired direct to output. You can install a switch to give you that option (bypass all controls). Introducing any resistance will load the pickup (dampen the resonant peak), but generally 500K pots seem to be a pretty good option if you want some control. 1M is brighter, and no-load pots are like direct wiring. You can mess around with different options to dial it in, but direct to output is the brightest, and easy to test - if you like that sound, work with it...
Another trick would be to use a low-capacitance cable. Introducing capacitance shifts the resonant peak down. Unless you use active electronics, the cable is the major source of capacitance. However, this would also affect your "single coil" tone. Alternatively, you could use a cap (or a few) to dial the tone in different settings.
The last passive option is introducing an inductor (like Wilde Q-filter), which wired on a tone control would gradually lower the inductance, and thus raise the resonant frequency. You'd also lose some volume (not unlke a parallel wiring on a humbucker, I guess). The trick seems to be not to dial it in too much, otherwise you start cutting lows and low mids. I have no experience with inductors, but theory and reports from others confirm this works. But given that it's not more popular, I assume it may not sound too good, or it may be too finicky to dial in.
In all this, I'm assuming Alumatones work like normal pickups, and that the "humbucker" mode is like a series mode.
