the I2S on 10P FPC is routed to the 40PIN GPIO header, which is designed for VIMs SBC. The 10P FPC connector not make too much sense on KTB and is removed on next gen product.
Yes, theoretically, the DAC can use the I2S from both below:
- VIMs SBC
- XMOS (USB to I2S)
But the time we define the Tone Board there are only VIM1 and VIM2 and both the I2S of these two SBCs can only work as master mode, which means not suitable for Hi-Fi application, that’s the reason why we:
- Only route I2S of VIMs SBC to 10P FPC via the 40P GPIO header
- Route the DAC to XMOS I2S
But things changed when the Edge-V and VIM3(L) and KTB2 are ready, but I’m still not allowed to post further details of KTB2 here at the moment 
We haven’t tried the XMOS I2S bypass, but I think it might be have issue that the mode(slave or master) of MCLK in some applictions.
The I2S of XMOS can be both input or output, but we designed for output on KTB for DIY users, and with multi-channels features with extra DAC expanding, for example:
- Stack with 2 more KTB, can be 5.1
- Stack with 3 more KTB, can be 7.1
PS, you might wonder why we designed one more screw holes on KTB:
Have fun!