Hi Fujak! Hi everyone!
I am using a similar setup. I use JPLAY in a double PC setup with W2K12 R2 Core, both with Phil´s optimization (Josef, Marcin and Phil have done a REALLY good job). Currently I use a StelloU3 as DDC into M7 via AES input. I am very satisfied for the moment and M7 keeps going better and better. If I were you I would try this way as well, try a good DDC instead of modding AGD USB32. Apart from SQ, you will be able to take the most of JPlay if DDC driver is Thseycon or similar. In my setup I run everything at minimum latency without any issue (Ultrastream, US=20, directlink...) via AES.
Let me ask you a question, have you tested you HiFace directly (no MUTECs into the chain) into SPDIF input on M7 (Input3) at 24/192?. When I connect the Stello to SPDIF Input3 on M7 I get some "electrical noise" that DO NOT turn up when using AES. Other Jplay user who owns a M7 as well, told me that he has got the same result with a different DDC when connecting via spdif cable into input3. Here I have read people using Audiophilleo, but this case is not similar to mine because it doesn´t need an spdif cable.
I have tried different cables but no luck in input3. It would be interesting to know your results. I imagine you do not get these noises in your current setup
I am considering this MUTEC as well, but I am not sure, may be you want to sell one!!!????