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

OP did you keep the old tubes? Usually 24-48 hours after they have been moved or shipped. That is usually the deal. So they get burned in and then they are shipped and placed in service. Usually after 48 hours they are ready for normal service. Shipping and moving is a valve killer..

Take a pencil and tap the tubes with the volume down.. You might be surprised.

R&R the new one with an old one UNTILL you  get the new valve. Or XChange the set.. 5 minutes I'm up and running with my amps. Mac, Cary, VTL, DIY

Regards

OP did you keep the old tubes? Usually 24-48 hours after they have been moved or shipped. That is usually the deal. So they get burned in and then they are shipped and placed in service. Usually after 48 hours they are ready for normal service. Shipping and moving is a valve killer..

Take a pencil and tap the tubes with the volume down.. You might be surprised.

R&R the new one with an old one UNTILL you  get the new valve. Or XChange the set..

Thanks. So, it's a brand spanking new amp with new tubes aside from my 3 weeks of use (and it sounded great for the first 2 - 2.5 weeks!). I just tried swapping pre-amp tubes with each other and now my L/R balance is WAY better. R now just slightly louder than L, whereas before this swap, L was FAR louder than R. It was a 10db difference at my usual listening level. 

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