I am using the HM-31 and it works very well.
I had a friends Sony BDP-S550 for a long weekend and I ran it throught the HM-31 and I did not see any degradation of the picture at all.
You can connect three devices and have only one output to your display.
Port #1 is the priority port so which ever device you have connected to this port it automatically switches to that port when the unit is turned on.
You can manually select any port you want with either the button on the front of the unit or with the remote that comes with it.
I do not run any of my video through the 990 because this way everything can be used and the 990 doesn't have to be turned on.
A little more wife friendly that way.