It's highly unlikely that you would have done any subtle damage. In most cases the device has some protection to prevent damage, as Yamaha will have included a diode that costs about €0.01 and simply stops reverse voltages.
Some devices will work with the power either way round, but they are rare.
If there isn't any protection, a reverse supply is likely to cause damage instantly.
I've never seen anything be damaged by reverse polarity that shows up later. It's instant damage or no problem at all.