Hi @Alanbass
Yes, you can connect an external device through the aux in, and use it as a headphone practice amp. That is the way I used it a lot of the time at gigs, except the external device to the aux in was the PA mixer instead of a phone/computer. I'm no audiophile, but I was happy with the headphone amp, which sounds clear, doesn't introduce buzzing or other noise, and has enough power to go louder than is comfortable for my ears.