If you dig back about 5 pages, you'll see that I had the same experience using 1964Ears V.3s with this amp -- low hum when no music is playing, and during very quiet passages. It doesn't matter what gain the amp is set for, or what level the volume knob is set to -- hum is always the same.
It appears to be a feature of this amp. I picked an Op627, which didn't help. Seems to be always there , but only noticeable with very sensitive phones (my 1964Ears are 117db/mW). Just loud enough to be annoying.
I ended up getting a RSA Shadow, which I love.