Time for me to chime in on the deal I got from mp4nation: FiiO E5 + SoundMagic PL-30. Used from my two currently-used sources: a SONY portable AM-FM radio and the Line-Out of a 5.5G iMod, it's a truly enjoyable listen - with plenty of embedded "treat-to-the-ear" goodies. I really LOVE the sound of FM stereo radio, and, using the FiiO E5 with SoundMagic PL30, it is a portable FM radio-listener's dream setup. Plenty of detail, very spacious, no harshness with lots of the air-guitar and move-your-body-to-the-tunes groove that makes FM radio SO enjoyable to me. What's really nice about this rig is that now I have a small, light and mainly, inexpensive, rig that I can take with me in public places (like while waiting for my car to be serviced) to enjoy - and to NOT have to worry TOO much about an accident or theft.
With the E3, I would use the headphone out of the iMod (for the volume control). With the E5, I can use the line-out of the iMod. The iMod puts out a sufficiently strong signal, and the PL30 are sufficiently sensitive that I use the lower end of the volume range of the D5 to listen - and the difference in volume per "click" is fairly large. Still, I can always find a "click" on the E5 volume control where I can listen very enjoyably. Would I prefer to use my SR-71A and Yuin PK1? For SQ, yes - but not out in public (too much money involved). By the same token, out in public I'd be very unlikely to be using my iMod. However, the success of this trial leads me to think of a lower cost iPod or Sansa Fuze with the E5 and PL30 for an ideal public-use and very enjoyable portable rig. The small, flat profile of the E5 (relative to the larger and thicker E3) also suits the E5 quite a bit better for use with a small and inexpensive mp3 player.
About the E5 bass boost: it is really useful to have the choice of bass boost or not on the E5. First of all, the amount that the bass is boosted is both meaningful for listening and toward the subtle side. Whenever a song seems a bit lean, the bass boost brings it into the "Ahhhhh!" spectral balance. And for songs leaning a bit hard on the bass in the mix, turning it off yields a nicely more pleasingly listenable spectral mix. The presence of the bass boost switch makes a noticeable difference in my enjoyment of a wider selection of music. Yet, it is subtle enough that (thankfully) turning it off or on will never (to my ears) transform a song from too-little to too-much bass. It's a smaller step designed to take too-little or too-much bass enjoyably closer to "enough".
For the more usefully portable form factor, the volume control and the well-implemented bass boost switch, AND, most importantly, its basic SQ, and for how all of these features are so effective in increasing the enjoyment of my listening, I find the E5 to be well worth its price difference over the E3. I'd say that its basic SQ is on par with the E3. It is the additional, very usefully implemented, features of the E5 and its form factor which make the E5 well worth the increased price. This is the first amp (along with the PL30) that has me serious about an out-in-public portable listening rig.