First, in another application, I discovered that TOSlink optical connections have a good potential for sounding worse than digital Coax connections, supposedly because of the differences in quality between the sending a and receiving devices. The TOSlink optical output from my PC sounds exactly like you described, dull and lifeless. I just did't realize it until I started messing with digital coax connections. I read a few places that said its because integrated sound cards on a PC are added as an after thought, and are poorly designed and implemented, susceptible to clock jitter from all the electrical noise flowing through the mainboard and power supply. The design of the digital coax may be more immune to these issues.
I have my Audiolab 8300CD connected to my NAD C658 by digital cox, and I hear no different between this setup and just playing the CD using the 8300CD internal DAC.
When you said something was "wrong with ALL external digital inputs" did you really test the Coax input?