SR225 is pretty much going to be your best bet. The SR-225 can compare with the HD600, and even the HD650 if you don't like laid-back headphones. (I like all headphones, so it's doubly expensive for me)
The other alternative that is a strong performer for the money is an HD580 if you can find one. They're usually under $200 and they're basically an HD600 sans cable (it does make a difference switching between cables) and different grills.
I use Spotify Premium too, even at my level of rig. I can't really tell the difference between it and FLAC anyway unless it's very specific tracks.
Are you saying the HD-580 has a different cable than the HD-600? They look the same to me.
I highly suggest the HD-580. This is where it gets weird...the HD-600 or HD-580 with HD-600 grills is the very definition of NOT FUN. Sorry if that hurts anyone's feelings, but it just doesn't do it for me with stock cable etc.
I do however love the HD580, but it's just due to how it has better bass, more upper mids (less low mids) and more treble. Sounds clearer too. Even graphs reflect this. All the differences are minor and caused by the HD-580 grills.
I got an HD-580 for $160. Totally 100% happy with it. I was very surprised I liked it when HD-600 bores me to death.