How to tell the quality of a digital amp.

Hello,

I am new to the world of digital and I recently purchased a pair of Dynaudio Air20 active monitors that have digital amplification. These are relatively older amps (released in 2002 or so). I was wondering if the fast developing world of digital technology has rendered these amps obsolete. How does one discern the specs of a digital amp to compare them?

Here is a link to the Air20s specs:
http://www.dynaudioprofessional.com/en/air-series/monitors/air20/

I do notice that the digital inputs on the monitors yield a much cleaner sound than the analog inputs. I suspect this is because of the ADCs are not of the greatest quality.
I appreciate any advise.

Harish