This "backdoor" entry worked for my SBG6580 when I got locked out at one point.
Arris is of no help.
You have no idea how long I've been digging around looking for a way to get into my router...your second suggestion is the first and only user/pass combo that's actually worked. You're a godsend. Now if only I could get it to let me change the username...apparently I don't have permission to do that, which leaves me wondering who does...