I doubt if you will need a new battery every 2 months. This is just a recommendation if you don't want your battery to die unexpectedly. Feel free to push the battery level as far as you are comfortable. I think 3 bucks every few months is a cheap way to have piece of mind.
I originally suggested you see a local tech to save you the trouble of having to ship it to me for what I assumed was some basic set-up issues. But apparently you do not have a good local tech if he blamed the problem on the nut slot. Once you fret the string, the nut is irrelevant and has no effect on what you are hearing.
So I suggest you send the bass to us and let's see what is going on and take care of it for you. Please include all of your contact info and a description of the problem.
Here are my thoughts (a lot of it based on my knowledge of acoustic instruments).
If you took a new instrument and just put in storage for 25 years, it would sound like a new instrument. The question is not a matter of age, but a matter of how much the instrument has been played. I think the more they are played, the better they sound.