I am new to computer hi-fi. Yesterday I connected my new external DAC (Heed Dactilus III) to my MacPro (2006) and I also tried my iPad (2.gen) with a camera-connector-USB adaptor.
The amplifier (Eclipse A501) and speakers (Eclipse 508) were the same.
The music files were Apple Lossless files from iTunes.

There was a noticeable difference in the sound quality. The iPad-USB-DAC was much worse than the MacPro-USB-DAC combination.
I thought that USB signal should be identical in both scenarios.

Could someone help me explain why this difference between iPad and MacPro?

Thanks.
Lez