Hey Tony it could be numerous things. Have you listened from a distance when someone else plays your bass? It might sound weak from the players position, but is still projecting out. Since you got it from Upton I am assuming it is not a set-up issue.
No actually, I have never heard my bass being played by somebody else. Interesting....
You bought it in August and now it's dry winter. Wood moves, even plywood to some extent. So, it's not the string. Now it's time to take it to a luthier. I second the R MacIntosh suggestion. FWIW, he builds beautiful basses.
Sounds like a good idea. Anybody know the best way to get in contact with him?
I guess this also brings up the question of how often do I need to bring my bass to a luthier? Am I going to have to repeat this every winter and summer?