It may be one (or more) of many reasons and it's hard to diagnose what forces are working on it. It might be something new or a culmination of little forces over a long period of time.
In any event, it sounds like a nice old Kay that may not be worth trying a do-it-yourself approach... too many unknowns
If it were my bass, I'd probably take the strings off, drop the soundpost and get it to someone who has a lot of experience with the old big basses.
Check for a loose bass bar - it's not so uncommon on those old Kay basses. Top really caves in when the bar lets go!
Take the top off to fix it. . .
These basses have a following so (in the abstract) it might be worth fixing. It is a serious undertaking so Mike is right, take it to someone who knows what they are doing. A very knowledgeable person is James Condino (Kaybassrepair.com.). That might be a long drive for you. If you were in the Boston area (still a long drive), Volker Nahrmann would be a good person to see.
I'd go on line before getting out the pallet knife and vinegar.
You should also expect a bunch of other things to be wrong.
I have checked the bass bar and it looks tight. The sound post is sitting on circular wooden pads and looks to be in the proper place. I have had the neck off this bass to re-glue. I am in North central BC Canada so shipping is not an option. I have repaired several violins but never had to work on a laminate top. Any advice would be appreciated. I am having a hard time finding information on this type of problem so any leads would help. I would like to thank you and all the other members for being so helpful.
Charlie, wondering how it went with the Kay Bass? My wife has the exact bass and I am contemplating the exact problem/solution you describe! We’ve had this instrument many years, actually got it in B C and it has gradually gotten bad in the excessive humidity after moving it to Hawaii. In our case, the bass bar appears to be still attached all up and down, but the belly, bass bar and all are sunk about 1 1/2 inches. There is also deformation of the neck-body to the effect that it would seem to need a neck reset. A different, if related issue. The overall effect is poor tone, not sufficient down bearing on the bridge, etc. I’ve probably taken inch and a half plus off the bridge over the years.
I’m contemplating the re-arching of the top and making a new, stiffer bass bar. Hoping to do it with just the back off.
So, hoping to hear how your project is.