Does amp input impedance affect the loudness

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.

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.