The problem with the quality in digital music is the bit rate, downloads from apple is normaly some 192 Kbps, on your Ipod maybe less, I use only 320 Kbps and no downloads, but always buy my CDs and grab them, I use 320 Kbps to save space as I have some 6000 songs and groving on my Itunes, best would be a losless format, so its not a question of getting a good amp but more the format of your digital files.
A good amp will give a better quality, but if your files are low quality it wont help you much.
I've been playing my Ipod directly with my BL 8000, and over BS 3000, and over an Onkyo amp, with the amps you improve the sound a little, but to my ears (old ones) nearly no difference.
BL 6000 are very good speakers, so keep plaing your Ipod with them, and when you have the money in your pocket get some more B&O to step it up a little.