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

Big laugh @ pepper spray. HA.

So, I have done the unplugging/replugging and checked all controls.

Now, just tried swapping channels. Sent the perceived hotter L output to the R speaker, and the R output to L speaker. As expected, now the R speaker is the louder one. It's a difference of 10db at my usual listening level. Crazy. 

Next thing to try, I guess, is musical chairs with the tubes. I'm bummed though as these are brand new tubes.

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. 

It sounds like you burned in the tubes and found a dud.

Thanks. I hoped that ARC's factory burn-in test would have ID'd the bad apples before shipping, but I won't be too unhappy if the issue is as simple as a bad tube.

if it is new from a dealer then you can get a warranty replacement tube from ARC, assuming it is a tube issue. If you discover it is a tube, call ARC they will send you out a new one. Maybe even a new full set.

Yep, brand new. Thank you for this info. Will lug it to my dealer to suss out asap.

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!

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

Thank you. Luckily I have a great dealer and he got me sorted asap.

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

Best advice on the whole thread, and maybe the cheapest!!