As far as I am concerned, the whole "you'll never work in this town again" I would just see as "the company is in trouble if you leave". If you don't have any customers there are probably very few that actually care if you leave the startup. There are likely more people in your surroundings that would rather see you happy :)
The real dilemma you have to face is that you'd like to stay on your two co-founders' good side and keep working with them on other projects. Going with what you say it looks like you are on opposite sides here. They obviously don't want you to leave, while you want to leave. The only thing you can do here is talk to them, find a middle ground somehow. Maybe you can do consultancy work for them? Is there a chance that they might be better off if you are happy?
I like being happy. :)