There's no mention of either NTSC or PAL in any spec sheet.
This link leads to Onkyo and the rear view of the amp lacks the voltage selector.
http://www.onkyousa.com/model.cfm?m=TX-SR573&class=Receiver&p=s
The manual for the similar model SR575 says the North American model does not have the voltage selector. Too bad. Under the general specs for the variants of this family of receivers they list the following power specs. I have no idea where you have to go to buy one from the "Others" group.
Power Supply North American: AC 120 V, 60 Hz
European: AC 230 V, 50 Hz
Others: AC 120/220-240 V, 50/60 Hz
AC 220 V, 50/60 Hz
Some aliases for the SR575=
TX-SR505
TX-SR505E
TX-SR8550
TX-SR575
You could do what I did over 30 years ago. Join the Army, go overseas and buy one in the PX. Every HiFi product we bought in the PX in the 70's had the dual voltage feature.