Why not just use the old Mac as a music server? The DACs in the new Mac are not as good as those in the 2160 and you aren't gaining anything with the new Mac from a sound quality standpoint. The only time you are actually using the DACs in the Mac are when you have a 3.5 mm jack in use. Otherwise the USB should put straight undecoded data to the 2160 for it to decode using its DACs.
_________________________
Music system
Model 990/7500/Magnepan 1.6 QRs/Technics SL1200 MK2/Aperion S-12 Subwoofer/OWA3/Sony NS75H DVD
APC H15 Power Conditioner

TV System
Large Advent Loudspeakers/ Polk center/Monoprice surrounds/Panasonic Viera 42 inch/Onkyo HT-RC260/Sony BDP S590/Directv


Home Theater System
Onkyo PR-SC886/Outlaw 7125 Klipsch RF-82 L/R,RC-62 center, RB-35 SR/SL, BENQ HT1075, Outlaw LFM1-EX/OPPO BDP-83/Directv
Harmony ONE
Blue Jeans and Monoprice interconnects
APC H15 Power Conditioner