I don't think the preamp is particularly expensive, just well designed. It's still surprising how many active basses have cheap sounding preamps which have boom, honk and fizz controls instead of bass/mid/treble.
The weird thing is the Sire preamp apparently has a stupidly low bass control which boosts at 20hz yet it never gets too boomy unless you're silly with it, can wind it up a decent amount and it just fattens everything up without being overbearing.