Greg,
The SB4000 comes standard with passive electronics and it appears that it can be upgraded to active electronics, but didn't see where there was an option for it to be a switched passive/active setup. Because it is Carvin's custom shop, perhaps its available as an extra cost option.
Passive systems have passive style PUPs which typically have larger magnets to generate sufficient signal and feed the signal directly to the amp. Passive systems only allow for cutting of signal with the tone controls or the volume control (like my old Fender P bass).
Active systems can come in a couple of variations. Some have passive style PUPs, with the inherent tone of a passive pickup design, but the signal passes through an active preamp (battery power supply). The active preamp provides the increased tone shaping ability to not just cut signal but also boost the signal, providing a wider range of tonal variations. Some active only basses come set up like this because users want the characteristics of the passive PUPs with the tone shaping ability of the active preamp.
I believe that the switchable active/passive basses are also set up this way with the switch simply bypassing the active preamp and feeding the PUP signal directly to the amp. Sometimes switching requires some volume adjustments due to the different signal strength from the passive and the active electronics.
Other active basses have PUPs designed for the active circuitry and can only operate when the PUP signal is amplified by the battery powered preamplifier.
The combinations of PUPs/pickups/nonpickups are mind boggling. Best advice is to take a rig with a known sound and try them out. If you find a passive that works for you, they can pretty much relied upon to produce that sound, every time, but are somewhat restricted in the range of tones they can produce.
If the range of tones available from an active set up is what tickles your ear, then that is what you want to acquire. Just have batteries on hand.
The batteries in my MTD last a year and in the Tobias KillerB 6, about 6 - 8 months. Don't know if that is typical or not, but the cost of batteries for them doesn't break the bank.
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Steinberger NXT5 EUB, MTD 535 Fretless, Tobias Killer B6, 72 Fender Precision
Eden WT500, WTX1000N & WP100, QSC PL230
Eden D410XLT, D410XST, 215, Fender Bassman 10
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