I'm confused? If you aren't going to use 5.1, then you don't need to decode it? Correct? Even a pro logic unit with digital in should work since you only want two channel out.
What's the second optical for? Coax or USB may work better?
Also, you are probably compromising on DAC quality going for older units.
My advice: Scrap your pre-amp and get a really high quality 5-10 year old pre-pro with all the digital inputs/outputs you will ever need. They will most likely still have 24/96 processing with dual burr brown or crystal chips, remote, auto switching for signals and maybe even a 2 channel bypass. Anthem, B&K, Sunfire, Adcom just to name a few that can be had for no more than the DAC Magic.
What's the second optical for? Coax or USB may work better?
Also, you are probably compromising on DAC quality going for older units.
My advice: Scrap your pre-amp and get a really high quality 5-10 year old pre-pro with all the digital inputs/outputs you will ever need. They will most likely still have 24/96 processing with dual burr brown or crystal chips, remote, auto switching for signals and maybe even a 2 channel bypass. Anthem, B&K, Sunfire, Adcom just to name a few that can be had for no more than the DAC Magic.