Actually I had a similar problem with my ER-4Ps when I first got the amp.
The sound coming out was distorted and surely it was the plug issue (The 1/4 to 1/8 adapter I took with me on my trip to China was not the best and the connection was loose).
This is not the case here as I use a pretty good cable with a good 1/4 plug.
What I did for now is followed EraserXIV's advice and turned windows master and wave volume up to the top and now using the volume knob on the amp.
It is now set about 10:30 and the volume is reasonably loud.
If I turn the volume knob all the way it is deafening which is good :)
The bass is a little too smooth , but I guess the amp still needs burning in before I can fully decide how it behaves with respect to HD600's
These are 300 Ohm after all.