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'. 

@temmple I’ll go back and expand on my original suggestion…

You have an $1800 integrated amp and a $1600 DAC. A $2200 power conditioner, upgraded cables or any tweaks be it network or vibration control is putting the cart in front of the horse.
Once again - start by upgrading the DAC. Next I would upgrade the amp. Source is critical. Amplification is critical. Don’t know what speakers you’re using.

Forget power conditioner or cables or network stuff. Not a worthwhile investment in the context of this system. Take your time and tackle it from the components perspective first so you can hear the real difference when you spend your money.