Hiya, glad this is well received. As it is a critical listening task it is fundamental to use a high quality audio source. It may not be the end of the world if you are 320kbps. As we all know extended top can go AWOL in some 128 files (steep LP filtering in some versions of MP3 codec) along with that swirling mushy blurred upper mid and highs. Bass tends to lose solidity and definition as well. Ultimately you can probably glean the basic tonal content from an mp3 but the clearer the reference the less ambiguity, and less ambiguity is where good sound is at.