That's odd... the pickups on my Thunderbird are much louder than the Casady. The Casady's low impedance pickup gives it that open, airy tone, but does give it a lower output.
This, really. I've got both. They're both wonderful, but very different. I personally prefer the Thunderbird, but it can't do that Casady thing, or vice versa.
Two very different basses. The only thing they have in common in the shape.
The cheaper bolt-on is a decent bass for the money, but it doesn't feel or sound much like a Thunderbird.
The Vintage Pro is a magnificent instrument.