Almost all brokerages have an insurance bond as well as the SIPC basic coverage. usually this bond covers 10 to 20 times the SIPC amount. Just call the customer service and ask them what backs your account... They will likely not know and have to transfer you to their manager (or try to BS their way through).... but they should know.
I think there's more of a chance of a trader blowing out his account trading than a US brokerage going out of business.