Usually I listen at 12:00 volume but I open the volume at max on one track and distortion were unbearable. I never had such a bad sound ever.
I swap every tube but same distortions. My speakers are Omega SAM
What could be?

Track 4 was from Jessica Young album "When I fall in love"

I'm saying I did not expect distortion at full power 2W with volume pot full open.

Your source puts out a voltage.  Typically a DVD player might put out 2 volts.  An amp is rated to not clip at it's max input voltage at full volume.  Steve's amps vary from amp to amp but my ZMA reaches full volume with a 2 volt input.  The CSP3 can push out 30 volts!  So, as I dial up my CSP3 past a 2 volt output, I have to dial back my ZMA pot from fully on.  At my current CSP3 settings and my TT source, I am pushing 4 to 5 volts into my ZMA so I have to "throttle back" my ZMA so it allows no more than 2 volts into it.  The ZMA pot goes from closed to fully open, like a faucet tap.  

I hope I've given a clear explanation but this is how I understand it.

Check the specs on your DAC, it should say what it's maximum output is.  Sometimes you can throttle back a source.  My OPPO DVD player has its own volume control, which I assume just regulates output voltage.

It is not listed for RME ADI-2
Only show as dB:
In the ADI2 DAC menu, the output is adjustable within this range
-5db
+ 1db
+ 7db
+13 db
Found this calculator but don't understand it...yet
http://www.sengpielaudio.com/calculator-db-volt.htm

Are you using the balanced or the RCA outs on the DAC?  There are presets that you can choose...which correspond to the numbers you posted.

Start on page 62 of the manual and go from there.

https://www.rme-audio.de/downloads/adi2dac_e.pdf

It looks like you may have to lock out the volume control.  This could based on whether you are using the balanced or the unbalanced signal output.  Based on the settings, clipping the amp would happen if your signal is too hot.

Dom