As you would need to plug the O2 into the SB-Z's headphone output jack, you would be doubling amping (may or may not affect audio quality).
Where as if you sold off the HD558s, you could get some AKG K612 Pro headphones, 114 GBP.
I just feel that once you have the SB-Z, it's a waste of cash to buy the EPH-02, unless you plan on buying low impedance headphones, like 32-Ohms or less.
Thanks for your patience and help.
I'm really leaning for O2 + Asus xonar dx, since i think it's a better option for long term use, and also O2 will provide me a better experience with its lowest output impedance (feel free to correct me if I'm wrong.)
Hmm selling 558's it's not a chance for now, since I only have them for less than a week..If only I knew I could have bought a new superlux 681 from a friend of mine (I know I can't compare comfort between these two but since it has lower ohms maybe it was a better bet? )
Thanks in advance,