Since a DAC is a digital-to-analog converter, there can only be 1 DAC in the audio chain. You only need to convert the signal to analog once. The only reason you would use another DAC with something like the U3 is because the U3 isn't actually acting as a DAC if you simply use it to send out a DIGITAL signal. The U3's only role in that setup is to decode the Dolby Digital 5.1 and then encode it into Dolby Headphone. That signal still needs to be converted to analog though, so you use a DAC with toslink input afterwards to convert it.
Your PX5's toslink OUT is digital, but it's probably just a simple passthrough (check the manual), so there's no benefit in trying to run a signal through it to another device/DAC. Running a toslink through it is no different than skipping it altogether. It's just there purely for convenience.
The 1/8" jack on the PX5 is analog out. So it's already gone through digital-to-analog (DAC) conversion. You can't run it to an external DAC, because it's already analog and DAC's only take digital inputs (otherwise they can't do their D to A job).
Edited by chicolom - 11/14/13 at 12:59am