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?
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I received the USB cable and rigged it today. I must confess that i was pleasantly surprised with the sonic improvement.  I can't talk all that audiophile lango, but i assuredly heard recordings in a different way.. the nuances of drums rolls, horn flutters, gasps of breath even.  Pretty remarkable.   My conclusion is that purchasing a $350 digital cable for a $5,000 headphone amplifier is justifiable.   Makes me wonder if spending the extra $300 for the next level cable would have been worth it?  And here i go down the rabbit hole :)   If you are curious, I'm pairing Foster MK II (closed) headphones with a MHA 100 Headphone amp and a MVP 901 DVD.  I have a pair of Soliloquy book shelves running to the 50W headphone amp but they get little usage.  Happy to be streaming-along better now.  Thanks for all the tips.
Since you have a Prized and Cherished USB Cable in use you might look into a Galvanic Isolation component for USB which is just this side of a dedicated streamer.
Ifi offers two nice USB related products: micro iUSB, and nano iGalvanic.
Though it would require another cable!
which selection of streamers might you recommend if i don’t require a dac? this was actually my next considerations. you’re right on point- thanks