My old wavelength audio DAC is still usable

Your Wavelength dac is older and doesn't support MQA files. The files use a form of compression and when they arrive at the computer they need to be "unpacked."

Because your Wavelength Dac is a good sounding well designed Dac with a good analog output stage. The Dac in the Node is compromised by its not very good analog output stage and overall noisy environment.

What digepix says is correct you would need an Dac that can decode/unfold  MQA files. No guarantee a Dac that does that would sound better than your Wavelength however.

@jond @digepix 
I listened the same song in both formats MQA and CD (24/96 I guess). 

1. MQA directly from RCA out of Node2 to CD in of my preamp.
2. 24/96 from Digital out of Node2 to Wavelength and to cd in of my pre.
3. MQA from Digital out of Node2 to Wavelength then to CD in of my preamp.

Results:
3 sounds best, then 2 second. 1 sound the worst.

Question: My Wavelength cannot decode the MQA, why I can get the best music production? I thought the digital out for my Node2 has to be MQA format, and it is not compatible to my DAC.

Thanks

The MQA files you are listening too aren't just MQA they are also new remasters so they may just sound better. Are you able to observe the sampling rate when you play the MQA's?

Are you even playing MQA files? :)