I'm new to the audiophile world and I was thinking of getting the Sennheiser HD 598 SR Open-Back Headphone and the massdrop O2 amp. I THINK the amp will accept the 598, but I'm not sure what gain I should get. I think it's the medium. My biggest question is that optional RCA cable. The 598 has a cord on it so I don't think I need that either. Unless it's to go from the amp to the PC itself, and it's giving me the option to buy it now cause I'll need one.