If I may offer some suggestions:
1) Don't stream during live performances. Download your content in an uncompressed format (compression for the web generally sounds like shit, and your live audience deserves far better), and stop using iTunes. It's not designed as a performance tool, it's for consumer grade listening of mostly mp3's and web-compressed streaming files. There are far better audio codecs out there.
2) Edit your downloaded content for sound quality, and use Ableton Live for live performances. That is EXACTLY the type of application that software is designed for. In Ableton, you can easily assign ins and outs to your K-Mix, and easily mix in Ableton with effects, etc, or mix directly in the K-Mix, assigning your backing tracks to your non-vocal and instrument lines (1-4), having the backing tracks assigned in Live to tracks 5-8, then in the K-Mix editor assign "Outputs: PRE" on those channels, and that will give you direct control through the K-Mix of all your levels, and no latency on your vocals or instruments.
Outputs 9+10 are the headphone outs. I don't recommend individually assigning to those except for very special circumstances, like if you were a drummer, and you were sending a click track to your headphones, or something along those lines.
Hope this helps.
BTW, audiohijack is great for ripping content to be edited, and so is JACK Audioserver, but I strongly advise against ever relying on streaming content for any sort of live performance. Your network connection could go down at any time, or you simply may not have coverage at all in your venue. Then what? Show's over? Good luck.