Kevin, since you mention the 'D-G' pickup, I'm going to assume you have the Alembic Activator P-Bass-style pickup.
IF that's the case:
It may be that they are not properly adjusted height wise in relation to the strings.
I would caution you to do this adjustment only when you next install new strings. Strings that have even only been played a little can already be decaying at different rates, you may have a bad one or a loud one, etc. So we want to remove that possibility.
Once your new strings are installed (and I'm also assuming that this is a four-string):
If this is a two-pickup bass (guessing there's either a Jazz style pickup at the bridge, or occasionally another P-pickup), run your neck volume full on and the bridge pickup full off. If the bass is a two pickup with a pan control, pan all the way to the P pickup. In other words, we want to isolate it as much as possible. If it's a single pickup P only, no problem.
And set whatever amp you are using to a flat tone response: Setting a big bass boost or a sharp midrange scoop will NOT be helpful doing this . . . all we want to hear right now is the sound of the pickups, straight off the bass.
Using a ruler (a little 6 with 16th's is fine), check the distance between the top of the pickup (the E-A one) and the bottom of the E-string with the bass in your lap in playing position. Whatever that measurement is, we want to adjust the other height screw on the E-A and then each of the height screws on the D-G to where you get that same distance all four places. This will then have the pickup faces the same distance away from each of the four strings, and should result in a more-or-less even response. Depending on your ears and the strings / string gauges installed, the D-G may seem a little bit softer, which is normal (smaller gauges, less metal) but not heard by everyone. In that case raise the D-G a 16th or even an 8th closer to really blend them. When you get through, generally the pickup will look like a roof on a house: High in the middle (under the A and D) and lower at the 'outsides' (under the E and G).
Now . . . .
IF this is a two pickup bass, you need to do the same thing under the bridge pickup. If it's a Jazz shape, make the distance from the top of the pickup face to the underside of the string the same distance under the E and the G (it's a one piece shell, can't worry about the A and D). Again the same proviso as above, you may want the G side a bit closer if necessary. Most pan-pot electronics never completely shutoff one side or the other when you pan to the opposite side, so this could color your picture as well.
If you do this right with new strings, it will probably fix this. Other causes could be setup or electronic which would require a visit to a GOOD guitar tech, not the Goth kid working behind the counter at your local Guitar Center. Wherever you live, use whoever the working pros use in your area, if required.
All the Best,
J o e y
