I have a Martin DXM acoustic guitar. The whole guitar body is made of HPL (high-pressure laminate). My question is, do I still have to humidify the guitar during the dry winter months (heating season), or is the HPL safe from the effects of low humidity?
I've never owned or worked on an HPL guitar, but I'm sure they are less prone to humidity issues than all wood guitars.
But, I believe the bracing is wood and there may be some other wood components, so I'm sure that low humidity can have an effect to some extent. I'd also be concerned about glue joints where bracing is glued to HPL.
You could experiment and see what it does during extended periods of low humidity, and find out first hand.
I have a plywood Brazilian made classical I've had since the early 70's . And I live in Minnesota where during the heating season the humidity level indoors can get as low as 10% and in the summer as high as 90% or more. I have noticed that the frets stick out on the fretboard in the winter and I don't notice them in the summer. The top has gotten to look like a lake on a windy day I don't know if thats from shrunken braces or broken ones as it does have a couple loose ones. But nothing has cracked or separated except where I broke it buy pushing in on the top. That was 15 or so years ago and the cracks have not spread open any farther. As for the waves on the top it has the smallest fan braces I've ever seen. I leave this out in a stand all the time as it is handy to pick up and play. That way I get more practicing in as getting my good guitar out of the case seems to keep me from just practicing as much. Take this as you my but I seldom see plywood guitar in need of crack or warp repair unless its been broken in some way
But I've seen a lot of plywood guitars that have bad or missing braces, which are usually hard wood.
The first guitar I noticed this with was a Martin 000CXE. Supposedly imprevious to everything, I noticed, like you Steve, that the bridge was starting to roll.
To cure it, I took the strings off so there was no pressure on the bridge, took a small can that would fit in the soundhole (small tuna-fish can size), filled it 1/2 way with warm water, and put it inside the guitar lying prone. Then I closed off the soundhole so the humidity was enclosed in the body, and left it that way for three weeks. The braces rehumidified, and the soundboard came back up, the bridge rolled back to where it belonged.
For what it's worth, I got this technique from Martin Customer Service when I bought a dry, sunken 000-15. Fixed it right up... BUT...
This is not something that cures overnight. It takes some time for the braces to REALLY rehumidify. Figure a few weeks.
Good Luck and keep us posted!