Thanks for the quick response.
My sofa is 0.5m from the back wall. I also think placing the 1723 subwoofer behind the sofa is best for two main reasons:
1. Enjoying high quality near field bass
2. Keeping the 1961 subwoofers in the front for stronger signal since they are connected by RCA and do not have XLR inputs.
Seems the way to go.