Top 2 strings

My bass gives me deep rich tones on the E and A strings, but not so rich on the D and G strings (almost tinny or brassy). Anybody know what may be the culprit? I've tilted the pickups one way then the other with nothing but volume changes. Will new strings do it, maybe? Anyone got a string set to recommend?
AxeArt

To achieve the "tone" you are looking for, #1 - start with a new set of strings - for a "warm" tone that it appears you are looking for, stainless steel will sound "brighter" than nickel wrapped, DRs Hex cores have a warm sound I have found, Fender/DR & others have "Tape Wound" that have a nice warm tone, finally - flatwounds may be the way to go to get what you are looking for.
#2 - Experiment with your EQ - even if you only have a traditional single tone knob on your bass & Low/Mid/High on your amp. I have typically always ran my bass wide open - Vol/Tone all the way up on a passive bass - using the p/u pan (think Jazz bass) to roll between neck & bridge to dial in a tone that suits. For my active basses (musicman bongo HH) I usually keep things flat sometimes boosting mids or highs to suit.
Most amps nowadays have a good EQ section - first try a little boost around 125-160K, roll off the highs after 4-5K, scopp the mids in-between but not much - a little EQ goes a long way. These are some general guidelines for a warm tone.
Remember to experiment - trial & error - that is how you are going to find that tone you are looking for. If your amp doesnt have a good EQ section, one affordable alternative is a graphic EQ stompbox like the Boss GEB-7.
The Quest for perfect tone is never ending!

Ditto to what was said above, the right strings for the right tone. E & A are also going to be in the lower range of your e.q. So if you like them leave the lower range (100-200) alone. So if you are waiting more depth in your D & G strings trying boosting your mids, 240-500 even up go 800 range. Another option is also a good compressor but this can some times do more damage than good if used improperly.

Try a thicker D and G.