Finally! I have found the culprit, it's my dac unit after all.
For the record, I have this dac http://www.ebay.com/itm/311726071214
I don't know what's wrong with it exactly, but when I insert the RCA outputs it doesn't output sound every time... I must put it in and out a lot of times and randomly there will be sound
Also when I get a connection success and I stop the music I can hear hiss and crackles, this amplifies my theory that it's just a poorly made unit (I tried with 2 different RCA cables and 2 different amplifiers).
So, a long story short this method works with our NAS, as long as one has a good enough dac