Accuphase E650 XLR Input issues

I'm asking on behalf of a dear friend, he has an Accuphase E650 which was used on RCA input without any issues, recently with an upgrade of the source, XLR was chose as the preferred connection.

It works fine initially, however just after few days, the left speaker start to sounded noticeably softer than the right, sometimes it normalise sometimes it stay on as is for long period of time, these were done as isolation steps:

- Continue to use RCA input, works consistently

- Swap left / right XLR input, still the left speaker softer noticeably

- Swap different DAC with XLR output, still the same behavior, left speaker softer noticeably

- Tried different XLR cables, still the same behavior, left speaker softer noticeably

- Tried both the XLR preamp input (CD-in, and Balance-in), same behavior, left speaker softer noticeably

Ran out of options, the challenges was the amp was purchased from an oversea distributor, getting local dealer support can be a hit and miss, trying to post this up to see if anyone ran into such issue before and also seeks some opinion.