I'm not sure I fully understand everything you are asking, but as far as using your Zodiac DAC goes, as long as you keep the signal digital until you get to the dac, you'll be OK. You won't be touching the dac in your eq.
With regards to the Toslink switcher, you may be able to eliminate it. If any of your sources have something other than a Toslink out (for digital), use that. If you have something other than Toslink on your sources, it will probably be an RCA/Spdif digital out. You can use it but you will have to get an rca to xlr adaptor. They make expensive ones, but I would try just getting some from a music store first. Unless you have very revealing equipment, the cheap ones should sound OK. If you have to make a choice on what source to use for the rca/xlr input, I would choose the more important source. If you end up plugging the 2 sources into the EQ, its very easy to switch back and forth using the controls on the face of it.