Probably too late, but I just got some 439s and had a similar thought - initially they're light on the bass.
Before burning in I noticed that the bass sounded great but it was a little quiet. I was considering a portable amp to boost up the bass, but decided to let them burn in first. It was a good choice, because 10 hours of listening later, the bass has really opened up and is now at an optimal level for me (I like a lot of bass as long as it isn't boomy, mixing with mids too much, or overpowering the mids/highs). The bass on the 439s doesn't do any of those bad things. It's clean and strong, although it does lack punchiness. I think the Fiio E11 is the best affordable amp out there right now, especially if you want to use the bass boost. The E6 would be sufficient for flat EQ boosting, but the bass boost extends up into low/mid and even mid frequencies, giving your music a thicker, boomier sound.
As far as simply improving the sound quality and increasing your volume range, the Fiio E6 is cheaper (~$26) and smaller than the E11, and using it on the flat EQ setting would probably do exactly what you're looking for.
Another note, I'm running them through either my laptop or iPhone 3gs, and they sound about the same through either one. I run at only about 75% volume on the iPhone and it's almost loud enough to hurt my ears.
Edited by hotKarl - 4/10/13 at 4:35pm