I mix down to stereo, and also use both an MP3 player (on iPad now) and an RC50 looper, some of the RC tracks are created on the looper, others are created in Sonar as regular tracks and carved into loops. One thing I have found for backing tracks is to *mix* the tracks but not master them in terms of compression - keep it light. I use a mixer for the track, loop and live sources, and apply compression at the masters (I have a StudioLive 16:4:2 with compression everywhere, but you can certainly get any number of small mixers and use an outboard compressor). This way you get consistent dynamics, and get to glue the sources together, and lose that "playing over a cd approach. For the same reason I keep the tracks dry too, except for a little deliberate snare verb, and apply verb to the live mix.
btw ForScore is a great app for iPad - as well as handling all your sheet music (lyrics in my case), you can associate any song with a track - very handy.
bunches of videos / music here
http://grifbamaisin.com Cheers
Grif
Sonar X1D expanded
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Presonus StudioLive 16:4:2
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