I've used the Zoom G1xon for headphone practice for many years. I turn the volume "Level" down in the "Patch Settings", so I don't overload the headphones. The answer is here in the Zoom G1xon User Manual on page 9 :https://www.zoom.co.jp/sites/default/files/products/downloads/pdfs/E_G1on_G1Xon_2.pdf

I was wondering if any members have built this interesting preamp pedal, it looks interesting.
http://www.instructables.com/id/Portable-Guitar-Preamp/?ALLSTEPS
Thanks
GuyB