Insurance is the best way to go. I use MusicPro but you can research other options. Ask questions, read every line, make sure you're covered for fire/weather damage as well as theft.
Take photos of everything and upload a gear list with serial numbers to the cloud (My MusicPro account has all this stuff saved there just in case.) You could also keep a gear list with your will or other crucial legal documents, in the hands of a third party as an added layer of protection.
As others mentioned, a safe is unlikely to protect wood from fire damage. As for theft, sure a 1,200 lbs. safe might keep someone from snatching them at night, but you're more likely to have them grabbed off the stand at a gig or out of your trunk while loading stuff.
In the end, even the nicest bass in the world is replaceable. The music is in your hands, anyway. The fancy basses just make it easier to get out, and sure, they're great to have, but I'd rather get my cat and my family photos out of a fire than my basses.
Invest in quality insurance, good cases/bags to protect your gear while traveling, good stands to reduce the odds of it falling over, and if you're super paranoid, stage said cases/bags near your primary egress route should you need to exit your home quickly. A good home security system, a motion sensor light, and a camera or three are going to offer you far more protection than a safe.