FWIW I went the whole hog on a dedicated supply in my old house and it made not a jot of difference.
I think a lot will depend on how good the existing wiring is. The main motivation for me to get a dedicated spur was to stop clicks over the speakers every time someone switched on a wall light in the sitting room. The improvement in sound quality was really a bonus. Initially the earth for the hifi consumer unit piggy backed off the existing one. When I later changed this so that both earths went right back to the meter, the whole sound became clearer. Just because some people hear no difference is, in my view, no reason not to try it. It’s cheaper than a Powerline and is likely to give a bigger improvement. At the very least, you know that the system has firm foundations.
Michael & HH...The reason I may not be able to install a dedicated supply, is because the CU is miles away from where the black boxes are going. Whereas, in my previous house it was very close.
There can no doubt, putting a dedicated supply which is purely for the black boxes, will be great for the system. However, I have to put my hand on my heart and say, that I when I installed it in my previous house, I personally couldn't hear any improvement in the SQ. Perhaps it did, but I couldn't hear it.
Not wanting to turn this into another 'dedicated mains' thread, because as we know, there are dozens of them. I just wanted to try and establish, if not having a dedicated supply, would be an injustice to the Power-Lines. As people have suggested, perhaps try them and see.