I have the Sennheiser HD600, Neumann NDH-30, Audeze LCD-X.
I use these with my SSL2 hp out, and with Qudelix 5K.
I spoke to someone about my impressions and his first question was: which dac/amp? and when I told, he just winked.
So my question is: is the difference this obvious with a more expensive dac/amp combo? Should I spend another 6-800 euro to get one? Do your headphones open up/perform that much better?
That depends. DAC wise, unless you want to nitpick about details no human being can hear unless they measure it, then they do. Amp wise, you have to ask yourself if the quality improvement is actually real or just the Fletcher-Munson Effect making you think like so because the amp can give you more volume.