I used the HD500 as an input source using USB to record a partially finished song. But I'll be changing that to coaxial input into the audiofire once I can get the cable. I assume SPDIF is much less noisy than an unbalanced 1/4"?
http://line6.com/community/thread/58902
I have very little chances to record with spdif before since the MD ages 10 years back. so I can't say much about my experience with SPDIF.
SPDIF would be "cleaner" then. but seems to be spdif does have issues of its own. so if I were you i'd just go XLR easier. mixing/mastering already is one hell of a headache. no need to add more since the trouble-to-clean-signal-ratio is not very justified.
The audiofire does have a clock sync feature, but it does seem to be quite a lot of trouble, more things to consider. Thx a lot guys, you all gave me lots of food for thought.