i have the same problem on my first bass.
there are two ways of fixing this:
1) removing mass from the head - which is okay if your bass is okay and not too lightweight
2) adding mass to the other side, e.g. the bridge. a badass bridge weighs more than a pound (i think 550 grams) and will do a good job improving the sound.
which bass do you play? if it's a relly low-priced one you should consider get another low-priced, but better one

my "problem-bass" is my first one, i have it for 6 or 7 years now and still didnt get it fixed because I don't play it anymore.
a badass would be around 150-200 €, same would go for lightweight hipshot tuners.
for 300 € you'd get some seriously good basses without balance problems.
$ 0.02
all of my basses were bargains