Just an anecdote about the importance of an isolation rack. When I bought my Ayre QX-5 many years ago I auditioned it fairly extensively at the dealer and was mightily impressed. It presented a beautiful sound aesthetic and the best I had heard at that point.
When I got it home however I was surprised to find a really noticeable (mechanical) transformer hum, especially when I listened via headphones with the Ayre only a couple of feet away on the desk in my study. It was clearly audible over the music in the quiet passages.
Took it back to the dealer and we set it up again. Hum gone. This is weird! It took a little while to realise that it was his Finite Elemente Pagode rack that was successfully grounding all the noise.
Long saga ensued with correspondence back-and-forth with Boulder, which culminated over a year later with them sending a revised power supply board that could be retro-fitted. Hum gone completely. Happy customer.
But that Finite Elemente* rack made quite an impression!
'* and the sharp-eyed may have noticed that I use their Cerapuc footers under the Ktêmas.