Subs for music only starts making sense when your spend more than $10/mo on new music.
Which I do not do...ever. My iTunes bill is usually more than £10 a month, but 75% of that will be 2 or 3 movie rentals. I can't see the worth in a music subscription service right now. If I like a song, I'll buy it and listen to it obsessively until I'm sick of it. I couldn't justify paying a fixed monthly price when I might not listen to £10 worth of music in a particular month, yet I still have to pay for it.
I better model might be a 'pay as you go' model. Say, you pay £5 as a minimum fee for any given month. Then if you pass a certain number of song plays/downloads, that fee increases until it reaches a maximum. Then you pay more or less depending on your consumption habits in a given month.