I'm running the S/PDIF output of an AWE32 through an external DAC. I guess the answer is mostly subjective (and perhaps even more so, given the AWE64's purportedly better analog stage), but I think it might be worthwhile "upgrade," especially when coupled with the fact that the AWE64 has synth and PCM digital-out capabilities...
I'm not sure it helps, but here are some comparison recordings:
http://queststudios.com/smf/index.php/topic,2 … 2.html#msg25022
The "Onboard DAC" and "Digital" EMU8000 examples are probably the most applicable to your scenario...