OK. There's a LOT to sort out here. First let me say that this can be done and I will give you some directions. However, I am in a hotel in Florida right now, doing this from my laptop. I am armed with the back of my hotel receipt where I have drawn out your intended routing, and the DM 3200 PDF manual. So, I might miss something, but I don't think so.
Before getting started, I want you to know that the labels "INP" and "RTN" on the inputs are confusing. "INP" here, in the routing/input page, means just what you think...What is the input for this channel? Enter it here. HOWEVER..."RTN" is not a send of that channel. "RTN" in the channel inputs is meant to offer you a place to have an alternate input in the case of going from tracking to mixing, where, assumedly you would no longer want to have external inputs, like microphones, coming in; you would rather want your raw recorded tracks coming in, like from your DAW, or tape deck, allowing you to mix. The action of swapping the Channel inputs from "INP" to "RTN" is accomplished by batch selecting 8 channels at a time, at the bottom middle of the input page, and then moving the curser over to "SRC Invert" on the left and hitting enter. If you have already set up your RTN inputs for each of the channels to accept SLOT 1, TRKs 1-32, then you can now mix down to a stereo output in the DM. It's not required, but this is how you would normally mix Out of the box "OTB". Many of us mix right in our DAW, and simply use the stereo send from there as our monitoring source. It's up to you, and either way works. But, that's what the RTN parts of the channel inputs are for, generally speaking, of course.
OK, on to the nitty gritty....
You want to create an effects loop in sonar, via spdif, through your MX300. Piece of cake. You have to remove the idea in your head that you have to put the daw's effect sends on input channels for this to work...you don't. You can create the loop without any fader action at all. This way, you have the full signal going out to your 300, and back in from your 300 into Sonar.
Step 1: FW channels. Set up Sonar to send a stereo output for your effect send via a pair of FW SLOT 1 Tracks. It really doesn't matter which you choose, although for a stereo signal you should use an odd number for the left channel and an adjacent even number for the right channel. Just for the sake of this, let’s choose SLOT1OUT-TRK 31 for the left side and SLOT1OUT-TRK32 for the right side. You will also need to select inputs for your effects return in SONAR, so select SLOT1IN-TRK31 and SLOT1IN-TRK32.
Step 2: On DM go to Routing/OUTPUT Tab--(yes, I did say Output tab because we are going to bypass any inputs here. We want to take this signal directly to the digital outs).
To move within the page, you will be using your arrow keys to move the cursor and the jog wheel to change selections and the enter key to make your selection final.
Step 3: On the output page, start on the right side at SOURCE SELECT. Choose SLOT 1 (I am assuming that your FW card is in SLOT 1 here.)
Step 4: Go to the middle of the page and under TERMINAL SELECT, select Dig OUT.
Step 5: Now on the left side, look under DIGITAL OUT, and you will have 4 spots to choose from. Spot 1 and 2 are for DigOUT1 L and R. Spot 3 and 4 are for DigOUT2 L and R. Since you said that your spdif is set up under Digital 2, we will use spot 3 and 4 for DigOUT2 L and R.
Step 6: Move the cursor to spot 3 and, using your Jog Wheel, turn it until you see "SLOT1 TRK31". Hit enter. Now, go down one more spot and do the same for spot 3, but make this one, "SLOT1 TRK32" Hit enter.
You now should see your MX300's meters moving in response to anything you may have assigned to your Sonar Effect send in this loop. Now, let’s get the new and improved signal out of the 300 and back into the DM, and on to Sonar's returns.
Step 1: Staying on the Routing pages, select "OUTPUT SLOT" since we are going to use the slot1 FW Tracks to back into SONAR.
Step 2: Under Source Select, select your source...DIN (digital IN...We’ll sort out DIN1 or DIN2 in a minute)
Step 3: Under Terminal Select, select your terminal....SLOT1. Now since we are using TRKS 31 and 32 for this example, our terminal will actually be SLOT1- TRK 25-32.
Step 4: On the left side, under OUTPUT SELECT you will see SLOT1 TRK 25-32. There are 8 spots here. Spot 1 is for SLOT1's output over TRK 25. Spot 2 is for SLOT1's output over TRK 26 and so on to spot 8, which will be for SLOT1 TRK 32. So...move the cursor to spot 7, jog wheel it until you see DIN2-L. Now do the same for spot 8, which will be DIN2-R. Hit enter for each one.
Last step: Now go to SONAR and check your setup. You should see meters running in the effects return.
That should do it...enjoy,
Steve