#2
AUX IN on your Boss is to allow you to plug in a music source like an ipod and jam with it over headphones. It is not an output signal you can run to the input of your amp. Use Amp output to run a signal to your amp. When in doubt RTFM
https://static.roland.com/assets/media/pdf/ME-80_egfispd03_W.pdf
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY
#4
It will be fine (presuming you're using a proper guitar-level signal output on the ME-80, should be the "Gt. AMP OUTPUT") but yes it will not sound good at all, at least in a Hi-Fi sense. Really I would just hook up non-isolating headphones (Apple iPhone earbuds come to mind) to the source of the backing track and play on the guitar amp normally. Or even a small mixing console/Audio Interface, micing the amp. OR even just use the ME-80 by itself with the backing tracks running in.

I discourage using the AC15 as both the guitar amp and the track amp because likely the sound will be discouraging. The amp will respond differently, likely will overdrive too easily, and generally be not a fun time.
Last edited by Will Lane at Aug 24, 2016,