Does amp input impedance affect the loudness

Mfb33,

600 Ohms output impedance is low enough compared to 21KOhms input impedance - it is more than 30 times less.
Thus, electrically 600 Ohms will not load down the amp & you will not need to tinker with the volume control vs. a 50KOhms input imp. amp.

You shouldn't hear any difference, but if you do it would be a roll off of the highs that you would be most likely to detect first. But, a 600 ohm output into a 21K amp should be just fine. An increase in volume would not correct the problem created by an impedence mismatch.