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

Hi all. Purchased a brand new Audio Research i/50 integrated amp a few weeks ago. Sounded fantastic straight out of the box. Left town for a week, came back and the left channel is now sounding substantially louder than the right. I have Vandersteen VLR CT’s paired with it.

The amp also has a built-in headphone amp. I’ve checked that, and the L/R balance is perfect.

Does anyone have a clue here what could be happening?

Thanks so much for everyone’s time.