the a900 and the hd595 are very similar, and both easily driven by the ipod. if you want a dryer sound I would go with the hd595 (it is also slightly more detailed), if you want something more lush go with the a900. the sr225 I havent heard in a long while but it is also quite similar to these headphones (aside from a decided lack of soundstage), and you probably wouldnt go wrong with them. you really have to audition these three unless there is a deciding factor (ie isolation). I would go with the a900s personally. I find the hd595 and sr225 uncomfortable, and isolation is a plus.
I'd much rather have my SR-325's for rock than the HD595's I've heard. Even when well amped, I thought the 595's were boring, kind of like a clearer version of under-amped HD600's. That said, I've never heard A900's or SR-225's, so I can't really compare the three.
I have owned A900 briefly and I wouldn't recommend it for you due to your music genre preferences. They are decent un-amped, fair cans for vocals, huge in size.
I have no auditioned/owned the other two cans you listed, I suspect both of them would need decent/dedicated amplification to really shine. For more involving rock experience, I personally would go with Grados.
SR60 is great, but you won't be very satisfied having it as your only headphones.