I bought a PA2V2 for the HD-25s for my iphone and seriously it is so worth the trouble carrying it about! At first i was like it doesnt make a difference but it is when you listen to it for a few days then plug it back into the iphone 4 (or ipod) is when you notice the difference. The bass is alot more full, vocals are alot more cleaner and forward. IMHO either get the PA2V2,Fiio E7, E17,or E10. You wont regret it.