You'll have to excuse my comment slightly, as I have heard the 501 but not the 503. I'm sure the 503 is an improvement. I felt the 501 left the music flat and unexciting. I felt it was more neutral than what I heard out of both the Sonos and Bluesound internal DACs, but just wasn't very "forward," if I can use that term.
The Mytek provides an exceptional depth of soundstage and has all those other bells and whistles, including full MQA and a useful phono pre.
After reading more about the 503 I am intrigued and would like to have a listen. I wouldn't expect it to beat the Mytek, but it's also half the price. It looks like a real nice unit. That being said, if it has the same sound sig that the 501 has, it's not my bag - but that doesn't mean it shouldn't be yours.
UD-501:
http://www.teac.com/product/ud-501/
UD-503:
http://www.teac.com/product/ud-503/