Im using headphones that are 35ohms. I can listen to them with the iPod volume full blast. Although slightly uncomfortable, it isn't "ridiculously" loud. Im not really worried about volume though. Im looking to gain more in the way of sound quality.
I have searched the forums a bunch of times, but I'm looking for specific gains with the gear specified. The amp would be something along the lines of a fiio. The headphones I'm currently using on my commute are Skullcandy Aviators.