When I did that, there was no sound whatsoever coming from the amp - only the phone setting gave any sound at all (through the phone). I solved this by putting my TT output through the headphone jack to the amp instead of using the amp out. It works.
I'm having this issue too. I have an Amplifi TT running into my Fender Blues Jr. The guitar stuff all works fine, but when I play music (using the Amplifi amp on my HTC M8), no sound comes out whatsoever. The app shows the music as playing (track time starts counting), but there's no audio. Any solutions to this? The Amplifi TT is fully updated, I've restarted my phone, etc.