Good question they're asking by the way : "Is anybody using a humidifier to store their bass?"

I usually see people having problems with too much humidity for instruments (or gear in general), but not enough .. ?
Will the wood develop problem if the atmosphere is too dry ? And if yes what is too dry, then ?
Wet wood expands. That's why necks start buzzing in summer, when the fretboard expands faster than the neck and causes the neck to flatten out. If it expands evenly (like on a well made instrument), you will only need to adjust the truss rod to compensate. If it expands a LOT, cracks can open up.
Dry wood contracts (shrinks). That's why a neck develops high action in winter, requiring you to tighten the truss rod to compensate. If it contracts a lot, frets can start popping out. Also, glue joints can loosen up or even fail.
How dry is too dry? Partly depends on the particular wood species, and what degree of extreme dryness it has already experienced in its aging process on the builder's shelf. There is almost certainly not a single magic number you can use. Some basses will probably be fine at 30% or 25%. Some may start coming apart at 25%. Someone in Arizona can probably tell us what happens (or doesn't happen) at crazy low humidities. I'm curious about that myself. I do instinctively think that a lot of what an instrument can tolerate is determined by not just the species of wood and the build quality, but also by the degree of extremes that the wood has already experienced and survived. And it's an organic material, so one piece is not the same as the next.
I am using a humidifier here (in the winter), in addition to the dehumidifier (in the summer).
We have gas-fired forced hot air heat. Without the equipment, the humidity would be 15% to 20% in the winter and 80% to 85% in the summer. Both of those extremes would probably spell disaster for the instruments.
When I first moved here, I had a small department store grade dehumidifier. It ran continuously, but wasn't enough for the square footage. The belly bowed on an acoustic guitar so much that the bridge started lifting around the edges. I replaced that unit with a Fral FDK70 dehumidifier, and summers are great now - it never goes above 50%. And for the winter, I installed this wall-mounted humidifier which I found at NAMM last year, made by TAC Water & Misting Systems.
Tom, the owner ("TAC" are his initials) also convinced me to feed it with Reverse Osmosis water instead of plain tap water, to guarantee that there would be zero dust, and zero mineral buildup in the machine. We have a well, and all our other humidifiers require frequent descaling, which is a huge hassle. I am very glad that I took his advice and got a Reverse Osmosis filter from him as well. The whole setup works fantastic, and I can keep the place at 50% humidity year round, which keeps all the instruments very happy. Never have to touch a truss rod. Hardly ever even have to touch a tuner in fact. And both the humidifier and dehumidier are completely self-sufficient, maintenance free - set-it-and-forget-it. (Like comedian Steven Wright said - sometimes I like to turn them both on at the same time and watch them duke it out.) LOL
I know it's summer (at least in the Northern hemisphere), so talking about humidifiers is kind of like talking about down jackets and snow blowers. But it doesn't hurt to plan ahead. For all the living space in our house, we have Aprilaire humidifiers on the ductwork on our furnace, and in each bedroom we use a Venta Airwasher. The Venta units are fantastic because they use virtually no power, and use natural evaporation so they cannot overhumidify and they don't cause dust to build up all over everything. HOWEVER, you have to manually refill them every day. And if you don't use distilled water or Reverse Osmosis water, then you have to frequently clean the mineral buildup from the machine using vinegar or equivalent. It was a real pain until we got the Reverse Osmosis unit and started using that water to fill a pitcher and carry the water upstairs to fill up the Venta units.
True story: I once played two and a half songs while standing outdoors (to get some fresh air because the cigarette smoke was literally killing me) at a bar in Talkeetna, Alaska when it was 24 degrees below zero Fahrenheit. I had to come back inside when my fingers just wouldn't move any more. I could hear the drummer fine from outside, but couldn't really hear myself at all. The band said that I went WAY out of tune while I was outside, but when I came back inside, I came back to proper pitch pretty quickly. I'm guessing the humidity was below 10%. That was one well built Ibanez, it didn't even get any finish checking.