First of all, I don't have too much experience in hands on electronics stuff, but you make a good point. If both sides show the same problem, then may be the issue arises in the signal path between the two volume pots and the headphone input jack, because before that, the two channels are supposed to be completely discrete. If you open up the chassis, I think the first thing to check is loose parts likes screws that are dislodged between two electrical contacts (or a contact and the chassis) and making a short or ground connection. Also, using a multimeter, check for continuity between different contacts that are not ground and the chassis. Again, focus on the front section near the volume pot first. I have never opened my amp, but there are circuit diagrams of LF339 available, if that helps. Post #22 Post #1570 I think, you may get a lot of helpful tips by posting a question in the DIY forum: https://www.head-fi.org/forums/diy-do-it-yourself-discussions.6/ Bypass path could be created, if there is say a loose piece of solder lodged between the L/R terminals of the neutrik jack. Also, if something came loose during shipping, it might be dislodged if you tap the top and sides of the amp (not too hard, and remove all tubes first), and the problem might be resolved. I am sure, they tested each amp before shipping, so hopefully this is just a small issue.