I think anytime you fill out a new subscription it adds on to what you already have. After I read your post I went to go check my account, last month I had 11 issues left, now I have 34. So basically i'm set for the next 3 years.
So to answer you, yeah.......they stack on from the looks of it.
Thanks to the OP for sharing the link, I signed up under you.
Whenever i'm online, I just leave the thing on and rate the music once the loop is done playing. On top of that, the Fame Guage adds points to your total faster. I've been using it for about 2 days now and I have around 1000pts...