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

Ask your partner if they been messing with you FIRST. I get out the pepper spray and wave it all around.

Unplug your headphones all the way.. 

Check all your controls

Check all your connection. That all takes 5 minutes. Unplug and then plug ALL the way back in.

New gear 101, check everything 10 times. Then again the next day 10 more times. LOL

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.

I'm bummed though as these are brand new tubes.

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

Not familiar with that particular ARC amp, 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.

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.