[TF2]

I am looking for a new pair of headphones, right now I'm coming from a pair of HD595s and I love the clarity of them but the lack of bass is making want to upgrade so I have narrowed it down to the Beyerdynamic DT770s/80 Pro, DT990, or Denon D2000 (I have also read great things about the HFI 780, but I need a comfortable pair).  I plan on using the headphones for metal (In Flames, Killswitch Engage, Devil Wears Prada, As I Lay Dying), movies, and other (alexisonfire, Audioslave, City and Colour).  I do not have an amp, my source is an iPhone 2 or my desktop with a stock soundcard.  I have enough for either but I'd like to stay as cheap as possible with great sound quality.  Also, I don't have too much to spend on an amp, would a Fiio E5 suffice for any of these headphones? 

So, I guess my question is:
-DT770/80, DT990, or Denon D2000 (or ultrasone)
-Which amp (cheap as possible)
-Why for both

Thanks in advance!