For fear of showing my ignorance I havent posted before but here goes.

You input 150mv and turn the rotel up to max volume and you get 1v out. You need 500mv to get max power out of the power amp. So inputting 1v might in theory overload the power amp.

But the preamp volume control adjusts the output voltage, so you will only get 1v at maximum volume. Its unlikley that you will be using the preamp (Rotel) at maximum volume so it wont be outputing 1 volt.

BUT what source outputs 150mv ? I am struggling here, I guess some old viintage gear might. 300mv to 500mv is more common and 2-3v for dacs and cds players etc.

The 150mv input 1volt output is merely a guide showing how much gain the pre amp provides. feed in 300mv and at max volume you will get 2 volt out. Use a cd that outputs at 2v and the preamp will output 13v.

But all this is with the volume control at maximum. Turn the volume control down and the voltage drops.

If the sound is really harsh, shrill, agressive, distorted then you might be overloading somewhere, probably on the digital sources only, but my guess is all is fine and you are getting hung up over the specs in the manual when you dont need to.