SPDIF question

[Camel]

I am currently mixing completely ITB and would love to incorporate an analog compressor and EQ in my work on the 2bus. Now, I've done a lot of reading on the subject of D/A conversion etc. and see that some converters have usb input but others work with SPDIF or AED.

My macbook doesn't have an SPDIF output or input (for printing the processed signal back into my DAW) but I can find some usb powered adapters which can output my signal as SPDIF. Wanting to remain in the 24-bit domain, the units that can do this seem quite pricey.

Will these adapters **** up the quality of my signal? Wouldn't it be better to stick with a usb/firewire D/A converter, both for the price and quality?
Or am I missing some easier/cheaper approach to solve the situation?

Thanks for taking the time to read and answer.