I have only practical experience and my comment applies only to amps with gain knobs (like the OP's amp) where no...and I mean zero...signal gets anywhere in the amp when the gain is all the way down (rare in home audio, common in pro audio), thereby rendering the amp safe for changing input sources, which I've done thousands of times without issue (I am extremely lucky however)...wear and tear on the jacks are also a non issue unless you're pretty clumsy or really obsessed with constant switching out and in. Grounding issues will raise their ugly head even if you turned the amp off first, so there's that. Also, regarding listening for differences...it's possible to switch things around and hear differences immediately and decide if it's good or not so good...although living with things for subtle in depth consideration is always a good idea unless the auditioned item sucks too badly as life is short...all a matter of degree. Have fun in your frenetic input damaging preamp testing voltage mismatch explosion session!
