The best advice I ever got for playing wireless music was from Tom here on this forum and it was to lose the Mac client to feed my network.
I bought a Synology NAS based on his advice and it solved most of the jitter problems I was having.
I too have Comcast as an internet provider and I have an Apple Airport Extreme for a router. I was using a combination of airport expresses and web tvs to distribute my music to my stereos. That was nothing but 1 big head ache after another. I had nothing but jitter and drop outs. Everytime I wanted to sit down and relax the gxx dxxx apple tvs had to download a gxx dxxx fxxxn firmware patch. You know how long those take. 15 flippin minutes. So...that shiet is gone.
Wanna run flac or alac files from your mac? You have to download amarra or jriver or pure music or fidelio. They take a ton of memory to run, they want money every time the offer a new version...so they are gone for me as well
I'm still using con cast as an IP and I've had no issues with bandwidth. I am still using my Airport Extreme for a router. I do like that since I am network config challenged. My Synology NAS is connected directly to my Airport Etreme so I don't need any computers on to listen to music
The thing that fixed all my drop out and jitter issues was moving to a sonos connect
http://www.sonos.com/shop/products/connectThey are a bit pricey at $350 a pop, I have picked a couple off ebay for under $280. They are nice, you just plug them in, push a button and they work. They seem to stand up a separate wireless network. I can stream music to my garage which is probably 50 yards away from my house. The Apple network doesn't even reach that far.
Just giving you some food for thought. I am liking this Tidal thing. It's like having access to a bunch of Flac files I don't have...and it integrates with the Sonos. It was very easy to use.
Before I spent any money on SW or memory for you mac...I'd take a hard look at trying a sonos. How many systems do you have?