It's possible, but are those strings at the same height as the others? Usually, when a bass is setup, the heavier strings have a bit more string height to compensate for their lower tension (sometimes as much as 10 pounds less than the G or D string). What frets are they buzzing at, specifically? Did you read the tutorial yet? The Gary Willis site I mentioned before is
here; it mentions a fretless, but the concept is still the same. It will give you a better idea of how these things affect your bass.
Usually string height is set with this procedure; the highest and lowest strings are set, and then a radius gauge is used to set the other strings to match the "curve" of the fingerboard, using the outer strings as a guide. Again, read the first page that I mentioned... it's all there.
If you still think it's neck relief, check it this way: press down the very first and last frets, and look at the gap at the middle (usually the 10th or 12th) fret. There should be a little space in there between the fret and the string, even though the other frets are pressed. How much gap you need is up for debate; I'm sure some others will chime in and give their opinions.