When I go look at a piece of gear, I request the seller let me open up the hood so I can look for burn marks, capacitor leaks, and solder joint breaches. I test each channel with a small speaker that I bring along with me.
No worries about rated power vs speaker power rating, unless you get really stupid crazy with the volume knob. Set a max power on volume and max volume on your preamp and eliminate all worries.
Bill