I love Carl's threads.
Isnt the sparrow the bottom of the line audio gd? As highly as audio-gd are raved about in the forums, I can imagine it losing to say.... an stx...
Personally I've only found amping to make a significant difference with a portion of headphones that I owned, in fact until I upgraded my source, I found that my rather expensive amp made my grados sound WORSE than through my sound card.
With my hd650 and my dt880 however, I have heard what I consider to be a significant difference by amping. Funnily enough, people hark on about how the hd650 loves tubes and power, but I found the difference between the hd650 on a rather "cheap" amp to be pretty similar to my pricier OTL amp.
As far as source goes, I have found that it has made the biggest change to my system. Sometimes better, sometimes just different.
I reckon, go to canjam or the like, listen to some stupidly expensive setup that you have no intention of actually buying, ask if you can plug your headphones into a couple of them. If you still cant hear the difference in a well recorded song that has been ripped well (or an original) then, I say, sell everything, bar your ipod a pair of koss portapros and be done with headfi.