First off, thanks for the product for the DIY community. This makes tweaking and playing a reasonably affordable hobby! Also, great implementation using the LDRs!! They are a beast to work with. Can't imagine the time spent matching these things!
Regarding the LDR switching, I have the single ended version of the LDR3x:
1. I took the +5.0V from your on-board regulator coming out of the power terminal +5.0V and used this as power for the LDR diode. I 've powered the unit via a +12V linear regulator.
2. Series POT multiturn resistor from +5.0V to LDR diode (anode). Value was 500R set to 250R. Used one each for left and right channels. This of course is times 3 because of the 3 input selection coming from your board so 6 in total.
3. GND is not connected to the "bottom"/"negative/cathode of the LDR diode. This is left floating.
4. Connected the cathode of the LDR from each *channel* to the "In1","In2", "In3" of your board.
5. Paralleled all the Left channel source inputs after the LDR resistors (other side of course connected to source).
6. Paralleled all Right channels sources, same as above. This gives the switched L and R signals that go into RI and LI of your board.
7. GND of all sources are connected together and plumbed to AG to your board.
8. Took the DMM and monitored the resistor value of the LDR with no loads or sources connected. Tweaked the POT till each LDR resistance was 50 ohms. As close as I could get it. Use the SR3 version of LDR from Newark. Pulling around 16mA through the LDR diode.
9. Did not test for repeatability, temperature or anything else.
One thing to note is the LDR resistance does't change on a dime so there is some area while switching that the LDR is between sources and possibly lowish resistance. Seems to work fine at home right now though. I've tried 2 sources full ON and did some switching. Everything survived but I can not guarantee this will work for everyone; however, this may not be a big deal, just something I noticed. I made a custom PCB for myself that has LDR footprint and relay footprints in parallel just in case. You could easily do this without a PCB though.
Let me know if you need more information, sorry for the fast typing, a bit short on time, thanks again!