Hi, Sun.
What I do is run the RP250 output to the Jamman input. The Jamman out goes into the amph input. By plugging the guitar into the RP250 I can....
1. Play guitar w/ or w/o drums thru the Musicman amph.
2. Capture either or both on the looper along the way.
Playing along w/ the drums in time is like practicing w/ a metronome. It gets a bit tricky knowing when to hit the loop pedal to start the loop repeating. I still have trouble there. Practice makes it easier, but it's usually somewhat like a waltz........ONE,two,three,ONE,two,three,ONE,two, three-LOOP!
This makes for a flexible setup. You can loop a riff & practice it to a drumbeat, or vice/versa, and save both together to practice over when you're ready.
You could theoretically save a few drums @ varying BPM from garageband to your looper to get started. It would give you a platfom to play riffs or chords over, and you could layer more onto them as needed.
That should get you up and running to practice some of your favorite Sunvalley noodle ideas.:AOK