I recommend the Sennheiser Momentum over ears.  I've been using them as my portable cans for a couple years and love them.  Good bass levels as they are closed back and don't need an amp.  They're also very lightweight so I've had no problem using them on the go or when working out.  

No gain settings, nice thing is there is no EMI with the card so I doubt I'd need to replace it as a DAC but have been thinking of getting a Schiit Magni 2 or the Vali to see what having an external amp is like.  Would you recommend going that route or considering maybe an O2 as an amp?

With the recent resurgence of many people building their own desktop PCs again gives us the choice of using a pci soundcard or opting to use an external DAC/Amp combo.  For example when I first built my desktop I opted to get a Creative X-Fi Titanium HD for my soundcard to use for outputting to...