Having now passed 150 hours of use with the Matrix Audio SI-1 network isolator in my setup, I believe it safe to say that I’m past the worst of run-in fluctuations and that the sound has finally opened up with a close to full measure of tonal density, insight and stability in presentation.
However, as of only a few days ago I still lacked what I believed to be a more authentic richness and texture tonally while also missing a bit of immediacy, but once the SI-1 sailed past ~150 hours of use things have started to fall in place - impressively so, while addressing exactly what I felt lacking. Indeed I dare say getting at least 150 hours in the SI-1 is mandatory to get an idea of what this unit is really capable of, or rather what it facilitates sonically.
Others have already gotten into describing their impressions quite thoroughly here, and I’m not sure I can add anything worthwhile. However I would like to make it clear that the magnitude of improvement brought to light with the inclusion of the SI-1 in my setup context has surprised me, also in regards to the nature of improvement; it’s not what the SI-1 appears to bring into the "equation" with its own contribution, sonically speaking, but rather - to reiterate - what it facilitates in order to flesh out the fuller potential of your digital source. This, I believe, makes it all the more interesting.

