Audio File Quality And Size/Headphone Quality And Cost

So, I'm someone who enjoys nice, high-quality music but isn't particularly interested in a massive vinyl collection and the high-end turntable/speakers that would be needed to really use it, or $500 headphones for a lossless file collection. I've currently got some lower-end Sennheiser headphones that sound pretty darned nice, and I have a digital music collection that ranges between 256 kbps AAC files and Apple Lossless files for artists whose music I really love and who are into a lot of musical details and heavy production (i.e. U2).

I've done some research about this and there seems to be a lot of folks who advocate lossless audio files and high-grade headphones, which are prohibitive due to file size and equipment cost, and people who advocate going with iTunes-level files and standard audio gear because they say it's impossible to pick out high-quality audio without the super-pricey headphones/speakers. Is there anyone here who knows much about this? It is obviously easy to hear the difference between the iPod earbuds and the Sennheiser headphones, but I can't hear too much difference between a 320 kbps AAC file and a lossless file on the same headphones. So, in reality, how much more audio data is truly available to the listener with the really, really nice headphones? Is there any sort of audio quality difference between a more compressed Apple Lossless file and a wav or flac file? What are your personal opinions in general on such things? There are times when I truly enjoy laying back and getting lost in the construction and performance of the music, but I also recognize that I spend a lot of time listening to audio files while walking outdoors or riding the metro, which interferes with the quality of the listening. Thanks.