From the little I know, impedance-wise the xb600 and the v6 might be fine with an ipod:
As far as the notebook, I think it depends on the brand and model. My HP laptop has a not-so-good DAC/Amp combination. Measurably so:
I heard Mac notebooks are a bit better, but dunno.
You might want to invest on a DAC/Amp thingy depending on what you have. I have a little bit of experience with the UHA-6S which is fairly practical given it's size, and does a fairly good job. If considering an HD800 I know some people that like to pair it with even better and more expensive Amps such as the ones offered by Eddie Current such as the Super 7.