@liangx5 Glad to hear you ended up better than you started with the new firmware and Iris 12th with I2S.
I sold my TP and Iris due to the conflict between the two and because I had purchased another DAC. I recently purchased another DDC and am using SPDIF on BNC connectors. The BNC connection sounds fundamentally different than the USB from the TP or my new DAC and also similarly different from the I2S sound.
I wish I had tried the SPDIF before selling the Iris now. It took a few weeks for the BNC to settle to its best but I knew right away I preferred it to USB. The only downside is no DSD and max rate is 192.
I don't understand the differences between synchronous and asynchronous but I am now a believer that converting USB or Ethernet with a DDC is beneficial.