Well you did reduce the headroom by using 15 volt regulators instead of 18 volt. 7818 and 7918 are very common and easily available.
and you may have a bad solder connection or a bad plate resistor.
All you need is an O scope to track it down.
As far as the amp goes, I think you will experience power amp failure. There are some problems with the design, and a bunch of them have blown up.