Julian,

You didn't understand... the analogue signal that is sampled by the amp is the result of 16bit/44.1kHz signal. So, you only can have a much higher frequency than 44.1kHz if you upsample from it... In this case, that is done by the amp.

In practice, you may end up with two upsampling stages instead of just one if you use an upsampling CD player... if you don't like the first one (CD player) why add a second upsampling stage (digital amp)?