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

might be room acoustics... is the room and speaker placement and reflective surfaces symmetrical?

or else it may be the amp... try swapping tubes in one channel to the other, one at a time, see if the imbalance shifts

tighten, redo all wiring/connections

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.