I have a simple solution to your problem and it will cost you $26. I was always running into the same problem as I have multiple Sony mega storage CD units I use as transport.
Just a couple weeks ago I decided I am NOT buying anymore DAC units. In addition, I am not going to spend a ton of money on a DAC that has a couple extra optical ports. It would seem to me there should be a whole row of them on ANY half decent unit.
But anyway... Here you go. Hit Amazon, look up a LINKFOR digital toslink optical 4x1 switch. Btw.. LINKFOR is the brand new, that is not a typo..
So easy it is stupid! There are 4 optical inputs and (amazingly) 1 output. Just plug up anything you need to run to your external DAC , use the remote to turn this little unit on, select which port you need and wow- problem solved. I was able to free up a couple of DACS that I wasted money on for some other projects. I am running 3 CD decks and a streamer thru this thing and it works perfectly!!! Gee, all for $26 bucks and I love it. http://LiNKFOR Digital Toslink Optical 4x1 Switch with 3ft Optical