Mixing Voltages to get a bigger one?
While some regulators are designed to be able to be put in series, most are not. Given that you have two generic no-name 5v/usb phone chargers, that's not the right way to go in any case.

You could break one open, and hope for a clean board with labels, or a common regulator IC without it's markings removed, so you can look for a datasheet. Most of the time, the charger manufacturer goes with the reference design suggested by the regulator's manufacturer, making it easy to modify, but sometimes not.

The best case scenario would be to use an external boost regulator circuit, to bring that 5v up to 9v. You would want to choose high quality capacitors to minimize noise, as with a music oriented device like your amp, would not be ideal. Since it is designed for a 9v battery, current is not the concern, just the voltage. Or simply buying a 9v power supply designed for audio systems, to avoid any noise issues.