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?

MacBook Pro’s already have outstanding power supplies that include great power conditioning - which contributes to their stellar performance.

Your best bang for the Money would be a good Digital USB cable.   If your budget is limited visit Best Buy and get the  Audioquest Carbon; if not - look at the Wireworld Platinum Starlight 7USB 2.0 or the newer Starlight 8 USB 3.1.

McIntosh equipment is also stellar; so you are almost there as far as Sonics go.
Have you considered a USB to S/PDIF converter?

I use an old Macbook Pro, with Halide Bridge (USB to S/PDIF converter) to DAC.
All in one cabling is an advantage for me.
Also have an iFi USB Silencer.
At work have a M2Tech Hiface2, which was recommended to me.

I'm considering upgrading the Halide Bridge. It's limited to 24/96.
Although I have no 24/192 at present.
Also have

Have heard people talk very highly of Black Cat Silverstar Coax cable.
See if you can hear a difference if the Mac is on battery or not. If you do, you may need a replacement PS. If you can't, you are good. The real issue is ground loops and RF/EMI noise being caused by the PS.
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. 
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. 

Best upgrade for streaming? Ditch the laptop as a streamer. It's not made for that.

There are plenty of purpose built streamers out there, even much cheaper than a laptop. They also free your laptop for other usage, and look better. Some examples in budget: microRendu, SOtM sMS-200. Bluesound Node 2, and Cambridge CXn
I didn't realize that.   Do you get a an appreciable gain in performance from a range of $500 Streamer to $2000 streamer?  And i'm not necessarily in necessity of storage capacity 1T/ 2T, etc..  I'm more keen on sound as determining factor of value for purchase.
Generally yes, but it is all dependent on the rest of your system of course. 

The streamers I mentioned don't have any storage. They are just that, streamers. Not servers, no storage.
Thanks!  I'll check out those streamers.   i also have a DVD player from McIntosh, 901.  Do you know off hand if i can use it as a streamer? I looked at the owner manual, it says there is an ethernet option.
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