Please compare the following headphones

I would suggest

1) uncompressed files on your iPod
2) if a newer version, take advantage of line level out

under these circumstances, I am sure you will be able to discern differences in headphones

I have a pair of HD 600s and like them just fine - very transparent and airy - not so dynamic IMO but maybe that is just the way I am using them

good luck

Craig-I have the 600's with Moon Audio Blue Dragon cable and a ASL MG DT OTL Mk3 amp set up. I have used Grado 325, 325i, AKG 501 and HD650's all with this set up. I actually prefer the 600's to all of them. The 600 and 650's are not that much different. Although I have not heard the AKG 240/271 Studio's or Senn 280 pro you are asking about...I think they are probably not in the same league as the 600 series as you mention.

The AKG 501 is a nice sounding phone...it falls way short of the Senn's bass though, but is nicely transparent in the mids. It lacks the impact of the 600/650 or the Grado 325 series.

Its hard to beat the Senn 600 especially for the price and it mates well with the ASL. Run in OTL it gives a nice bloom to the sound.

The AKGs are very nice but I must say, if you can swing it, purchase the Sennheiser HD600s. I use mine with my iPod through the Headphone Section of my Cary SLI-80 and love them.

I have owned AKG's, Grados, and BeyerDynamics but the HD600s are really special. Not to mention, very comfortable.

Good Luck!

Chris