Mark, another reason for staying is your feelings on future prospects, where you are. Or at least that the potential is so much greater. That's an important factor for those who work in SME's and especially startups - where you know from the outset that you are underpaid for the typical 60-80 hours per week requirement.
And talking to people in startups from other fields, it's not just relative to the last corporate job that you had ... but the corporate equivalents to what you are doing now, which have been opened up by your startup opportunity if it's your first real crack at management. Remind yourself that in the latter case, it's a big IF since you have a chance to do stuff that people 2, 3 or even 4 levels up were doing at your old corporate job.
But if you feel positive about your firm's potential, that is wonderful. I remind myself every day, that the Big 3 EDA companies started with maybe a dozen people in the Bay Area and five more in Portland, Oregon.