Which is the best upgrade for the money for steaming from lap to through amp?

I am looking to upgrade my system with either a line conditioner or USB/Digital cable.  Am budgeted to spend about $500.

I am current running a mac pro to a McIntosh MHA-100 Amplifier and headphones out from amp.  

So where will I get the greatest benefit in transferring the sound quality to my Headphone?  Form the Line conditioner to the lap top and amp or from the upgraded cable from laptop to Amp?