All very interesting and thanks.
I already have Sennheiser HD580's and a Grado headphone amp, as well as a vintagey Headroom amp. I have Shure headphones, Etymotics headphones and Bose noise cancellation headphones.
For the iPod, my main concern is working out and long haul airplane flights.
For flights, I have just received a pair of the Sennheiser PXC-450s.
These seem to to have a higher build quality than Bose, more robust noise cancellation, and also seem to sound much better and more detailed.
Glad to know that the newer and higher storage capacity iPods have not cheapened the sound.