Depending on the initial stiffness of the wood, some instruments that are naturally too tight sound better when it is humid, and others are better when the weather is very dry. If your instrument has a better sound when it is humid, you can alleviate the problem in dry weather by inserting a humidifier. Dampit works. Case humidifiers only might prevent the pegs from shrinking and slipping down. The little two-inch tubes filled with water were originally made for things like cigar boxes. Once the case is closed, there is no air movement within the case. For everyday use, there is no way that enough humidified air will go through the f holes to the inside of the violin body, where it is really needed to prevent the plates from shrinking.
If you play the instrument a couple of times a week, all the humidity, which may have slowly accumulated in the case, disappears when you open the case. If you go on vacation for two weeks, a good case humidifier may help a little, though a dampit in each f hole is much more efficient for the inside of the violin body. There is not much you can do if the instrument sounds better in dry climate. Inserting small silicone bags for dryness into the f holes is not an option of course.
Post movements away from and closer to the bridge are more permanent in their effect. The further you move the post away from the bridge the more leverage you have for the bridge to move the top and the instrument may respond easier. But, if you move the post too far from the bridge you will loose power.
In our experience, the difference in thickness of the post has more effect on the sound. A slightly thicker post makes the sound darker, too thick will make the sound muffled, a thinner post creates a brighter sound, and too thin a post will create a shrill sound. The most appropriate thickness depends entirely on the individual instrument.
A very effective way to manipulate the sound is to experiment with different strings. Don’t go out and buy lots of different strings. If you are familiar enough with a particular dealer, ask them if they have some used g strings to compare to your other strings on the violin. It is easier to make a comparison to the neighboring d string as you play, rather than going by memory and play one set against another.
Finally, I would like to address another misconception. If there is a particular note on the violin, which does not sound good, is too weak, etc., you will not be able to improve the strength of this note in relation to the others by moving the sound post. If you get more power with an adjustment, the whole area gets louder but the individual note is still weaker than the notes above and below.
Don’t underestimate the power of suggestion, which can make you feel that the problem was resolved, but a few days later it miraculously comes back. It reminds me of the woman which was sawed apart by a magician right in front of me and was revived again just a few minutes later.