washburn status weak output on 'd' string

[mojobass]

recently got my hands on a washburn status, plays great but got one niggle - the output on the 'd' string is weaker than the other strings! Ive tried raising the bridge saddle to give greater string relief but no difference! one thing ive noticed is that the upper frets 1 - 4 appear quite 'flat' in comparison to the other frets could this be a contributing factor? I'm thinking of a refret but dont want to shell out if the end resulkt will be the same! anyone had sinilar problems? Ive got the action down as low as i can go and when depressing the 3rd fret theres buzz on the 'd' beyond that its fine! cheers