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

you need to get Audio Research involved.

And, buy thee a simple tube tester you will have for the rest of your life.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=tube+tester&_sacat=0&LH_TitleDesc=0&_odkw=accurate+tube+tester&_osacat=0

make sure it comes with instructions, list of settings for tube types.

Please call your ARC dealer.  I see so many threads here about new equipment where the OP never mentioned contacting the dealer. Why?  I would not contact the company first.  When a company sells through dealers, part of the margin is to field the customer service calls.  

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

op - wish you good luck, arc should and will take care of you

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.