Checked again, DX3 pro and SMSL m500 both could run hd600 properly. I still look for the best one in my budget.
Currently look like Atom is the best candidate. They have a little bit of channel imbalance though. Don't know if I could recognize this or not since I don't have a chance to test Atom.
What would improve after you change the volume knob?
Currently I only listen though my hp probook 450g1, mac & imac are in the office. So I am looking for a best dac, amp in my budget to improve the sound
You say the Atom is good. But that's only an amp, from the title, it seemed like you wanted a DAC + AMP. If you're going for bang for buck, then yeah, I am not understanding why the Atom is the best candidate seeing as how that's only half of your equation?
And no change a volume knob doesn't improve sound, you'd have to change the whole volume pot itself which will require soldering, and other considerations.
Also, you said your budget is $400. If that's the case, then the Atom + D50S is virtually the best performance for the price.