Here's the story: two months ago I purchased a Music Man Stingray on eBay, in perfect condition, and it's the nicest sounding bass I've ever heard. But lately I've been having some issues.

In my previous band we played in Drop C, and so I noticed that when I was in Drop C I would get some REALLY annoying fret buzz around frets 2-5, predominantly on the A and D strings. This issue did not exist until a couple weeks ago, mind you.

So after I quit my band, I tuned my bass to standard, and the problem was gone. However, in the past few days I've noticed the fret buzz happening in that area while in standard. It is absolutely insufferable.

Can someone help me figure out what I can do?
Spend money on a quality setup by a qualified technician?

The fact that you keep playing in different tunings, coupled with changes in weather and atmosphere has caused your neck to flex. It's a peice of wood, it does not always return to the same position as it was before.
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I wouldn't go jumping straight to any conclusions about neck damage. If your tuning low, its very common to get annoying fret rattle. If your planning on playing low in the future, grab some heavier gauge strings.

As for now, since the problem is persisting I would suggest either learning how to properly set up the bass yourself or seeking help from somebody that does. Quite frankly I think paying sombody $60 to setup a guitar is outrageous, becuase it is fairly simple to do.

On a final note, I want to point out that its almost impossible to completely eliminate fret buzz on bass guitars. Just about any bass that you pick up and play clean will have slight rattle to it in some areas. However, usually you can adjust your EQ or your pickups to dampen the sound so its not so noticeable.
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Maybe you need some relief in the neck. Too straight, and the 1st few frets buzz. Too much relief causes the high frets to buzz. Also, is your A,D strings compensated for the fingerboard radius??
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