I think I was a bit misunderstood....I will try to explain.
I said that Phonitor does NOT have black background sound signature, IMO. But, when I say black background, I am not thinking about noise floor. Noise floor itself is really fantastic on Phonitor (as you said even on +22dB gain everything is completely silent, although I didn't try any IEMs). What I meant under the term 'black background' is a specific sound signature which I have only found in my Astell&Kern SR35 DAP so far. There is a thread specifically dedicated to 'black background' sound signature here on the head-fi, it is here:
https://www.head-fi.org/threads/recommendation-for-a-dac-or-amp-with-black-background.967643/
In short, it is very hard to precisely define black background (like everything in this hobby) but here is the summary from the thread above:
Finally, Phonitor is a fantastic amp, no question about it. But it does NOT have black background sound signature as per the definitions given above. Its noise floor is great but black background is something else. I think Phonitor has some deep bass reverb which somehow fills the 'air' in the background (it is not noise!). It is very liquid sounding and that is probably the reason why someone who is get used to a tubes could like the Phonitor as well. So, there is no a 'void' space between the notes. The easiest way to test that is on some acoustic tracks. With Phonitor, a natural reverb from acoustic guitar would be more extended, thus filling the space. Maybe transients are a bit relaxed...