I have my sub set up in exactly the same way. Co-ax to my Onkyo, High level to the switched 'Sp 2' terminals on my Arcam.
Works perfectly well in both systems and has done without issue for over a year.
I'm careful to only have one source running the sub at a time, but even if I didn't, I think my sub gives priority to the low level connection, eliminating the 'clash of sources' problem.