I think most amps have protection against going under/over voltage input. For example an amp will shutoff if its power supply reaches under 10v. Im not sure about 15v though, i mean i car will usually run at 14.4v so its not THAT much of a difference.
There are modded amps out there like mine for example that can take 16v racing batteries to power them.
How would you even get an even 15v to an amp though?