DAC not working

I didn't quite catch the manner in which you have this stuff all hooked up, but are you sending a single source to two different preamps at the same time? This isn't necessarily a bad idea per se, but some (mainly older) preamps short out all the sources that aren't selected - this is to reduce crosstalk in their selector switches.

The logic is this: if it isn't selected, then you're not listening to it, right? So who cares if the signal is shorted? This of course doesn't anticipate another component trying to use the source at the same time.

You report that the Sony has a toslink optical output but that you hooked up the Sony to a DAC using a RCA cable.

Either your Sony has a SPDIF/Coaxial digital output (and not just the toslink as you stated) or you have hooked up your analog output of the Sony to a DAC. This won't work. Not sure what the DAC is saying it is locking onto, if in fact it is an analog signal. Check again, does your sony have a coaxial digital output? This is one output that carries both channels.

you then say that you can hook up the CD to either preamp and everything is fine. When you hook up the sony to the preamps, I am assuming you are first changing where the cables are attached to the Sony before hooking them up to the preamp. If you are just unplugging the RCA cable from the back of the DAC and plugging that same end into the preamp - then you are not running a digital signal to the DAC, you are definately running an analog signal.