Albert asks many questions that I had as well. It sounds more like a phase is out on the left side. This would make the right channel twice the level as the left.
I do not recommend anything other than a call to ARC first thing Monday morning. Ask to talk to Leonard. This man is so very helpful and patient. He has helped me with numerous question/concerns with my preamps/amps. For me it is very convenient to have them in my home town but this too was just coincidence.
As Albert points out, the LS5 is a true balanced product. I do not know if this has anything to do with its magic, but I have yet to hear a line stage bring on such an incredible wealth of ambience/ bloom and 3D. But then I have not heard the latest BAT and Hovland and ARC Ref 2. So far no other ARC line stage has this magic less perhaps the Ref2.
I did have the Mk II for 6 years then sold it in hopes of improvement but could not find anything near the price so I bought another 2 months later on the used market. And since then, I have sold that and gone with the MKIII. This has even greater resolution and a far more natural top end....the MK II has an exaggerated treble with an often annoying sibilance with some music. This is entirely gone with the MK III.
I want so much to try a shootout with the VK50SE but have just not had the opportunity to do this.
Anyway, swap some cables around and see if the problem stays with the same channel or did a single phase connection break off at a connector somewhere.
Good luck.
John