Yes, technically speaking, you just have to get the right kind of plug adapter that is rated for the current, but that's my response based on "technical" to your "technical question".
There is very little chance of that, I would say 99.99% no, it won't affect sound quality. The DC side of the power supply that the amplifier actually is powered would not change in any significantly way anyway.
The difference should be very minimal, if at all. The voltage difference is, on paper, 20%, the frequency difference is also 20%, so the current difference is going to be near 0, under the same load. In fact, Denon Japan rated the amp for both 50 and 60 Hz, that indicates to me (me only) the voltage is likely rated 100-120 V to begin with (again, that's just me educated guess, you have to ask Denon).
In terms of longevity, in general, 20% higher voltage is not going to stress devices rated for 100 V, such audio devices are not going to be designed precisely for 100 V anyway, obviously. You best bet is to ask Denon.jp, if you asked Denon.com, unless you manage to bypass their first level customer support rep, you would likely be told no, don't do it etc.. If I were the design engineer, I would never design such a power supply for an audio amplifier that can only be used for 100 V 50 Hz but not 120 V 60 Hz, that would be engineering malpractice, in my opinion.
You probably can, or at least be able to get the US version transformer, but if do such things you will likely void the warranty, so I would not suggest you do that.
Or you can get an external transformer to convert 120 V to 100 V.
All such efforts are most likely a waste of money and time, again, as mentioned earlier, if you look at the specifications in the manual you will see that it is rated for 60 Hz as well, that tells me the 100 V thing is also a non issue, that it likely has the same power supply but they didn't bother to specify it for 100-120 V, laziness, standardization, who knows? I can only take my own educated (as an EE) guess.
Bottom line,
@TLS Guy probably just tell you what he told you because that's the safe way. No one is going to tell you otherwise, without further details/qualifying conditions etc., because if something happens, one may feel responsible, be blamed or whatever..
So, my more perhaps positive response is based on technical, theory etc., and I would have no concern if I am the user (but I am an experienced EE), but I cannot/will not tell you to just go ahead and do it, and at the minimum, I would say again, contact Denon.jp for an official answer.
From the manual:
Note that it specifies 50/60 Hz!!
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