Experiencing speaker imbalance, while amp headphone output is fine. Any ideas?

Please call your ARC dealer.

Thanks. I did first thing this morning!

even well tested tubes, shipped, handled, packed/unpacked, can sometimes suffer from a version of infant mortality

You were right on the money. Turned out it was just a bad tube that lived only 3 weeks. Replaced today and all is well.

Thanks so much everyone for the replies and help!

Swap the speaker cables left to right. At the amp.

Tubes are a consumable.  If that doesn't set well with you, solid state was invented for guys like that.

I'm glad it worked out well for you.  It's very frustrating when equipment is not functioning properly, but when you're buying new from a dealer, part of what you are paying for is a warranty and insurance for your peace of mind that someone will be there to assist you when something goes wrong.  I recently had an issue with a new phono preamp. I emailed my dealer and within the hour I received a detailed email from the distributor that explained how to address the issue.  It would be nice if this could be done with used gear as well, but unfortunately it's much more unlikely.  

@gnarmite

glad you figured it out

did you take @oldhvymec ’s advice and get pepper spray too? he also keeps a fire extinguisher handy next to his listening chair too, as i understand it

great mechanics are always prepared... 😂👍