I've had different results from my PX5's trying different things to get all the audio from my receiver through my headphones without having to disconnect my speakers to accomplish this.
I have an old Denon 3806, and it's pretty good functionally, but I had to setup an 8x1 switch to link all my upconverted classic consoles like SNES, SS, Dreamcast, Xbox, Genesis, etc. up via HDMI cables to it. My goal was to get the PX5's so instead of having it annoy my wife with my speakers, to use the headphones to play all the consoles through. However, it seems as there is no way to get both HDMI and optical out with the receiver that I know of.
So next I tried a headphone plug and piped that into an RCA converter and plugged it into the back using the RCA inputs. The result was a very loud and annoying hum, which I'm not sure what or why it's causing it, but I can only assume it's something to do with the headphone jack signal not matching up very well with the input to the headphones.
Next I tried using the preamp out to put that into the headphones. That worked fine, but the volume controls are tied to the preamp out and thus I have to have both the speakers and the headphones active to get sound, and that didn't work to well, and have no way to 'turn off' the speakers.
My best case scenario is using the tosslink cable. I did find a way to split off the HDMI signal, but the only switch I've found that has optical out requires some sort of video signal coupled with the audio signal. That means for me to get optical out I'd have to get another converter to switch from HDMI to composite. I had to call Monoprice to verify that the switch would not actually work with a straight HDMI input and an optical only output. That made me sad.
Anyone have any suggestions?