Ok, back to the original topic.
I'd say it's doable.
There will be opamps used to amplify the signal. And there probably are ALOT of pin-compatible opamps that have better specs than the one used. Just find out a good one and replace it. You can know what components are used just by looking at the chips on the board. No need for a schematic. (Although you won't know where exactly it is located in the signal chain...) You can trace back the schematic. Now that's hard work, but if it's not to many layers on the pcb, it's doable...
For the convertors: same here probably, you just need to look what chip is used, and find a pin compatible substitute with better specs. Or, looking at the specs of the original chip, you can find out what signals come in where, find a chip that has better specs and takes about the same signals, meaning, you can get it to react in the right way (so it basically uses the same protocol to get it's data), then replace the chip, not by soldering it on the board, but by using tiny wires to connect the pins at the right places. This means, you don't really have to be pincompatible. Just connect it with wires, and stick it on it's place using a piece of rubber in between the chip and the board.
The firmware doesn't have to 'recognize' the hardware. DACs just get controlsignals, and don't talk back. So, you just have to make sure the DAC listens.
And the same for the mux and S&H parts if any of that is used...
Now, am I smart or what??
Does it show that I'm doing modifications on CD/DVD writers all the time? Getting signals out, getting them in our testboards,....?