sakahara, I'm not a MAC guy but I'd like to share my experience with you in building a PC music server. Over the last 4 months I've played around with multiple hardware configurations and a few basic rules have emerged
1) processing power is king
2) faster memory is a close second
3) SSD drive for your OS partition will make you smile
4) swap files, prefetch, indexing, all those cycle stealing background tasks are your enemy for musical transparency
How old is this iMac? Your initial comment makes me suspect many years old i.e. underpowered. I thought initially that I could use a 3 year old PC as my music server but when I compared the older, slower processor/memory machine to a newer faster machine it lacked the transparency of the faster machine with the same USB DAC. Are you able to try a faster MAC to see if that helps? Im pretty happy now with the proximity of my machine to CD, but like you I am looking for the right choice for USB digitalI/O since I distrust internal sound card digital output.
1) processing power is king
2) faster memory is a close second
3) SSD drive for your OS partition will make you smile
4) swap files, prefetch, indexing, all those cycle stealing background tasks are your enemy for musical transparency
How old is this iMac? Your initial comment makes me suspect many years old i.e. underpowered. I thought initially that I could use a 3 year old PC as my music server but when I compared the older, slower processor/memory machine to a newer faster machine it lacked the transparency of the faster machine with the same USB DAC. Are you able to try a faster MAC to see if that helps? Im pretty happy now with the proximity of my machine to CD, but like you I am looking for the right choice for USB digitalI/O since I distrust internal sound card digital output.