The way you worded it you had me thinking that you was gonna remove the astro mix amp from the chain and use a standalone headphone amp with the external dac or a all in one unit, while keeping the sound blaster Z in for gaming and connect that to the external dac thru digital. That's where my suggestions was coming from. If your gonna keep the astro then there no reason for a external dac at all especially if your gonna get a super cheap to use with your sound blaster Z, ince you will be better off using your Z analog out in that case. Unless your planning on removing your Z and using something like a Sabre U2 usb in it place which can be found cheap for $49 which has a combo analog/optical jack that you would use a 3.5 to tos-link adapter and connect it to your Astro mix amp.
When you originally did try the Z to the astro using digital, did you make sure you had the option in the advance tab check for sending stereo mix to spdif out. Which is the only way the Z will send it DSP functions over optical. Also did you set windows and games to 5.1 and kept the creative Z panel to headphone mode when you was using SBX surround. If you don't do this then it won't work correctly. It has to be done this way so The function can downmix the 5.1 to 2.0 for the headphone. Which why SBX surround work best set up this way, so it will greatly, which also applies to CMSS3D Headphone and Dolby headphone usage. Do not set spdif out on the Z as the default playback device, if you do this then the card won't be sending any of it features to your mix amp thru Digital.
