Here is something I tried on my 75 & it seems to do the trick & use the Amplifi's guitar speaker. on the Amplifi app I took everything out of the signal chain except the volume. I plugged in to my guitar input of my HD500 & ran the left mono output to the guitar input of the Amplifi. Bottom line is there are some sweet tones coming out of the Amplifi & I don't have to worry about hitting the 4 mid / hi horns with my guitar signal. The backing tracks I am using sound good & are not affected by this configuration. Try it, I think you'll like it.