Hi,
So in summary you are powering the Mini V3 by battery (connector 8) which has no electrical connection to mains or the speaker, and have an audio lead that connects the Mini V3 3.5mm socket (10) to the B&O Phono?
Can you confirm the connected pins are as follows: 3.5mm plug pin 1L (or pin 2R) to RCA pin 1, and 3.5mm pin 3 GND to RCA pin 2 GND?
If the above is true, have you tried plugging a CD Player or something similar into the B&O to see if the same problem exists? As you’ve tried headphones direct in the Mini V3 and powering it by battery I suspect the problem is associated at the B&O speaker end. I notice there is no decoupling capacitor in the B&O so so inserting a 2uF cap could be something to try next. The value won’t matter for the purposes of this test.
Simon.