The output impedance of 6.3 as reported here in Massdrop is when using the method that has maximized output level and 300ohms load, and the generally taken method is 1vrms output into 32ohms load. And under the 32ohms load it is 0.18 as posted by Shanling at:
http://www.head-fi.org/t/799595/shanling-m5-impressions-discussion-thread/75
http://www.head-fi.org/t/799595/shanling-m5-impressions-discussion-thread/75