The 2 Puritan products mentioned are definitely worth your consideration, but I would agree with Audphile1 re: upgraded PC's (if you haven't already of course). IMHO it's worth investing in to the network side of things: dedicated network switch, streaming and USB cable, or whatever you're using to connect your A6 to your Gustard, maybe a network filter such as those from Network Acoustics.
I have a streaming only system, and while I do have the PSM156, upgrading the network aspects of my system gave me a more tangible improvement. As always, your mileage may vary, as will others'.