Hi Solderon,
Well, I guess opening up the BM3400 (AGAIN) is no big deal - I'm getting quite used to it now!
I think you're probably right in it being a 1/4" (6.35mm) Cliff socket, but I couldn't find the exact model on their website.
This one has 12 pins on the rear - 6 for each of the left and right channels, but B&O used only 3 pins for each channel.
Pin allocations as follows (Left and Right Front Channels):
4 - Green (from Power Amp & to 220 Ohm resistor)
5 - Green White (To Speaker)
6 - Headphone via 220 Ohm Resistor
9 - Yellow (from Power Amp & to 220 Ohm resistor)
8 - Yellow White (To Speaker)
7 - Headphone via 220 Ohm Resistor
Regards
MY
Learn from the mistakes of others - you'll not live long enough to make them all yourself!