I've gone through quite a bit in terms of hardware to try and find the best sound for my headphones. Currently I have the Sennheiser HD600s. In their current state I would describe them as being accurate and neutral but not having quite enough highs as I would like and being "closed". They just don't feel as open as I think they should be. How do I know if it's the headphones or my amp / dac? 
 
I have the HT Omega eClaro , going off the stock hardware (no OP AMP replacement). I've heard that it's a decent soundcard for headphones / speakers but what could / should I look for to improve the sound of the headphones - or is it the headphones themselves? By upgrading , would I really improve the audio that much or is it generally considered that what you hear from the headphones is what would give you a general idea of the direction they would go for an upgrade...
 
When looking for an amp , does the size matter - portable or desktop? Would it be true that the full size amps would give better sound quality then a portable (like the Pico)? I still don't completely understand the need for an amp. Is it because cheaper / poorer quality hardware can't push your headphones ... in terms of loudness or does it actually open up the sound of the headphones themselves (or is the sound quality from the hardware of the amp / or the dac exclusively).
 
I have my hardware connected to a computer - is a DAC ever necessary ... when would it be needed , if the music playback hardware has a dac in it already?
 
 
I'm sorry if this doesn't make sense completely. I'm just trying to explain what I'm looking for / how to look for / etc. 
I would just like some help.