Most of the plug-in power supplies that say "DC" have just a half-wave or full-wave rectifier with NO filter capacitor. First try a 500uF 15V electrolytic capacitor wired across the Wall-Wart output (observe polarity). Or start with your existing DC plug-in, add the filter capacitor, and then add an LM317 IC chip regulator circuit between the filter cap and your amp.
Finally, there are 9V or 12V DC REGULATED power supplies out there that you can purchase ready made. Check the surplus electronics stores.