Thanks for the replies guys. What Nolitan just mentioned here is most likely the reason Accuphase does not see it fit to support it as an option board...
Here is more information I got from a forum from Taiwan:
ChrisH: I used to own Accuphase DG-28 along with AI-AD1(mm/mc phono stage) and AO-B2(balance output) many years back.No problem for you to use AI-AD1 as an analogue input and feed into your line Preamp via balance output.
Note that your analogue signal will be RIAA equalized and amplified through AI-D1 initially and then digitalized by DSP chips, digital signal goes to balance output board incorporates with 20-bit DAC and restores to analogue signal. The signal path seems weird or brings owls to Athens but you may have some fun to adjust the equalization other than RIAA.
Here is more information I got from a forum from Taiwan:
ChrisH: I used to own Accuphase DG-28 along with AI-AD1(mm/mc phono stage) and AO-B2(balance output) many years back.No problem for you to use AI-AD1 as an analogue input and feed into your line Preamp via balance output.
Note that your analogue signal will be RIAA equalized and amplified through AI-D1 initially and then digitalized by DSP chips, digital signal goes to balance output board incorporates with 20-bit DAC and restores to analogue signal. The signal path seems weird or brings owls to Athens but you may have some fun to adjust the equalization other than RIAA.