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?
stephentp
Thanks for the input... that's something to consider as well which i hadn't considered. Best!

After doing some searching, I'm going with your advise.  A silver starlight 7 USB.  Arrives tomorrow.  Very interested to see the affects it may have :)    I couldn't afford the platinum but i got the Silver used for about the same price as a new entry level cable. 
Friend
You should have a complete System after your Wireworld arrives!  With a
MacBook Pro and its already stellar design feeding the excellent Wireworld which will feed the McIntosh MHA100 - you should be in the house for quite awhile enjoying Music.