For a web developer, 31K is not a lot. You probably wouldn't have too difficult of a time replacing that level of pay, especially if you are good.
Key words for me were "cozy" and "super-simple." I would leap at something more challenging for more money.
As for screwing your old company, just let them know well ahead of time. The company will manage. I'm sure they have had many worse disasters.
It's important to consider stuff like: is this enough career advancement, do I need the money etc. Getting jobs for more money is not that hard if you would be looking atm, so other considerations outweigh the money.
Use the info you just gained to make your decision. I feel pretty strongly that the old guys will not give you a nearly 50% raise but some raise would be nice. The startup life is not for everyone. Keep that in mind.
If startup fails - I'm sure the old firm will take you back and even with a rise, because you'll have more experience. (I know some cases when people left one firm and then came back with a rise).