For others who only need the cheapest good portable amp:
I just got the Fiio E5 for use with my Sennheiser CX870. It does give the bass that extra punch with bass boost. With JetAudio Plus and DSP Manager on my Cyanogenmod Android phone I have a much better sounding set up than I did with my Cowon iAudio U3.
I experienced the crackling problem for the Fiio E5 while charging, but that's only because I used a different USB cable than the one provided--the one provided was too short. The original USB cable has a ferrite choke (that heavy cylindrical shell near the ends of some cables) and it actually blocks all the interference. The only problem left is the stereo cable receiving noise when plugged in my laptop so I ordered ferrite clips for that.
This is my first amp ever and I'm very satisfied with how the E5 turned out to be flawless for what it promised for $18. I didn't want to spend the $24 to get the E6 since I was aiming for the lowest possible price for my humble needs, and if only the $8 E3 were also rechargeable I would've gotten that instead but replacing batteries is too much of an inconvenience.
Great in-depth review of the E5 here [blogspot.com]
comparing it to the E6 [blogspot.com]