Davey wrote...

But I know that there is going to be a mismatch between the PC's output
and any input on the amplifier, as it is not designed to accept such an
input signal.

You shouldn't have any problems as long as you take it easy with the
volume control. In pre USB days this was the only practical way to get
audio from a PC to the hifi.

I'm currently feeding the output of a cheap and quite cheerful USB
audio dongle:
http://www.lindy.co.uk/usb-2-audio-adapter/42961.html
to a 70's JVC amplifier which has an input sensitivity of 150mV and
have not noted any mismatch or overload. More recent amps designed to
cope with the typical 2V output of CD players should have more
headroom.

--
Ken O'Meara