Puritan 136 vs 156 for simple system

@temmple - I actually have both. I started with the 136 and ended up buying the 156 too. Firstly, the Puritan product up-leveled my musical enjoyment in both of my systems. They are both solid products. 
 

For your system, listed above, I think a 136 would be great. The 136 gets you 90% there. The upgrade to the 156 is there, but subtle, and I think most folks would be absolutely happy with its sonic impact, while saving some money too. 

And later down the road I can upgrade to the Ultimate mains cable from them. 

My opinion - instead of dumping cash into power conditioner, upgrade your dac. Install a dedicated line if possible and add better power cords. In this order.
A power conditioner isn’t going to magically transform this system.

I had PSM156 and much prefer my components connected direct to wall outlets.

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'.