Actually, I don't think that's true. When wiring passive pickups into an active preamp, there are generally two ways to do it:
- pickup -> volume pot -> preamp -> output jack. In this configuration you would use typical pot values used with passive pickups; i.e. 250k or 500k
- pickup -> preamp -> volume pot -> output jack. With this setup the volume pots are typically of the type used for active circuits - 25k or 50k
I've wired basses both ways, and they both work fine, although the first option allows for an active/passive switch.
Having said that, however, the best recommendation is to look up the various wiring diagrams for the preamp you're interested in. I know EMG has lots of good info available on their website.