Visually, compare the string saddle heights -- you'd probably want the A to be the highest saddle, since you'd be matching the arc of the fretboard, so it's possible it was crudely set too high, and combined with and overly tilted pickup, the A ends up with the worst volume drop. And the nut may be cut poorly, giving the A even more of a high starting point...not as likely since the pickups are so close to the bridge, but everything could play a role.
If you want to experiment, try this -- warning -- this will necessitate resetting up the action later!: set all of the saddles at exactly the same height -- low or high, doesn't matter. Raise one of the pickups to be nice and modestly close to the strings. Now play them with even pressure, amp at volume, and compare their relative volumes. Mostly focus on the A-D comparison, I think, or maybe the E-A-D -- the B and G are very different in terms of pitch and string diameter, so might confuse matters.
In the end, the bass probably just needs a good setup -- new basses in the economic category often do.
Post your findings, and good luck!
ltt
