While some sound differences may arise from changes in the actual humidity content in the woods of an instrument, the majority of weaker or more compressed tone, increased wolf note awfulness, or whatever failings you're perceiving in your basses when humidity falls (heating systems generally dry out winter air indoors, even subfloor heating to some degree) are owing to tightening of the belly and back against the soundpost. A soundpost can't shrink with winter. Belly and back (at least with arched backs) try to get narrower (minimally however, with plywood basses) but as the ribs, especially along the bottom, prevent this narrowing, the arching gets flatter instead. That flattening squeezes the soundpost, often quite dramatically. I've seen many bass soundposts so tight that after taking off the string pressure then gently forcing the soundpost out, the belly has fallen 2mm or more. Ideally a soundpost should be fitted such that there is no descernible pressure raising the belly, so such cases demonstrate that the post was fitted in humid conditions and has been tightened by the dryness-triggered compression against the soundpost ends by belly and back.
Many years ago it was standard fare for musicians to drop by a luthier's shop and have seasonal soundpost adjustments to at least largely compensate for this. The same post can be used, but of course when humidity drops into the low 30% range the post would have to move towards the centre of the instrument owing to arching geometry. This would result in some loss of volume, especially in the lower registers. Some players would actually have 2 soundposts for just this reason, a winter and a summer soundpost. (On a related note, Gary Karr used to travel with 3 bridges of varying heights, as he didn't want bridge adjusters yet wanted to maintain consistent string heights in various climates - his adoption of an adjustable necked bass took away this necessity.) Watching a hygrometer, especially the modern digital hygrometers which are generally quite accurate, can easily tell the player when it's time for such an adjustment or even a soundpost change. But even without a hygrometer, common sense should tell us that when it's cold outside, the furnace is on, and it's probably time for an adjustment as humidity inside is dropping.
This past winter in my region hasn't been all that harsh, not many weeks below freezing really. Still, it has been something of a battle keeping humidity in the mid-30's in my instrument storage room with just a Sunbeam ultrasonic humidifier. For some of the time I've had to supplement that thing running at full blast with a large plastic storage tub full of water, and for a week when I was out of town last November and couldn't fill the humidifier, I hung a towel into the water tub with most of the wet towel exposed above it, acting as a wick. Checking my hygrometer which records highest and lowest humidity, I was relieved to find it had not dipped below 33%. At 30% nasty cracking sounds are virtually inevitable... and then the hunt is on among the 50+ violins and violas and the several doublebasses in that 9 foot by 11 foot room. I've not heard those sounds since one very dry day in the mid-1990's, before I was aware of the danger. An icy wind had come from the North, and three very fine antique basses started making noises like popcorn within an hour of the furnace kicking in. I put a stock pot of water on a hotplate near them and within an hour the noises had stopped, leaving me with a lot more repairs than the owners of those basses had anticipated, for which of course I did not charge. Likely the same sorts of cracks would have opened at their homes, but it happened on my watch. The next winter during a similar cold snap a bass which had never cracked in decades suddenly opened up along one rib while in transit to my home for a different reason, a mere hour at sub-30% RH was enough to stress the wood that much.
Take humidity seriously, compensate for it whenever possible, and avoid dampit type things as if filled enough to make even a little bit of difference they tend to drip and cause damage. The more recent microfiber bags of water seem to offer a superior portable solution, the entire outside surface delivering evaporating water to the inside of a bass or cello. The most effective home made solution I've seen for portable humidification is a pair of large sandwich ziploc bags with numerous holes punched using a paper punch, and these filled with good quality sponges, these improvised evaporators then inserted into each f-hole and hanging by well secured strings between the two. It's not ideal, still leaves the possibility of dripping damage, but if one is careful to wring them out and check them frequently and keeps the gig bag zipped up around the bass, they can do an adequate job. Of course once the bass is out at a venue there's not a lot to be done... so a winter soundpost seems a good idea to preserve at least most of your sound.
