Volume differences

Hello,
my problem is that my basses volume differs from string to string.
There is not much difference between the E and A-strings, but
there are som bigger differences between those to and the D- and G-strings. Some buddies of mine told me that they have exactly the same problem... Its hard to notice while playing alone, but its easy to hear when for example playing an E on the A- and playing an E on the D-string. Its absolutely different. The volum differs a lot. My question is that, could it be helped by changing the pickups or something?

 

Thanks guys ill try that