My main set of headphones at the moment are a set of Focal Listen One, my main source is an iPhone 5S which has a good DAC in it already or my MacBook Pro, which again has an Intel HD Audio chipset. I don't see the need to buy a DAC but I was thinking about buying a headphone preamp just to play with valves more than anything else. My other amplifier I have at home is an Onkyo TX-SR601 which I connect to by an Apple TV through its optical audio port. I then use DLNA with my iPhone as the originating source and listen to the output through the headphone jack when I'm plugged in.
My existing amp has a range of outputs including CD out, video out, monitor out line out, component out, optical out and so on, so connecting to a second stage is not an issue if I just wanted to use my existing solid state Hi-Fi receiver for amplification without colouring the sound.