David, I waded through considerable nonsense on the Simaudio site and a little bit more in the W5.3 manual to try to get an answer. The details given are very skimpy, but apparently they have available for purchase a bridging cable which would invert the polarity of the voltage in one channel so that it doesn't meet head-on with the voltage in the other channel when they reach the speaker. It suggests that the result would be a maximum of 700 watts into an 8 ohm load and makes no claim as to 4 ohm loads. Perhaps you should contact Simaudio or at least the dealer for more specific information.
The insight I'll offer at this point is first, forget the bi-wiring(aka buy-wiring), as this is electrically meaningless. As to bridging, this does indeed increase the maximum power available, but the 350 watt into 4 ohm spec in normal operation is already more than enough to drive M80s to beyond safe(to your hearing)listening levels.
