In my case, I have highly efficient field coil horns (~105dB) and a small room, so some background hiss was heard. When the 18NS is muted, there was no audible hiss from the listening position.
I resolved this with Shunyata grounding (ALTAIRA Signal Hub) and power supply filtering (Denali 6000 v2 and Typhon T2) and vibration cancellation for the field coil PSU (Seismion). No more audible hiss, even sitting 10’ away from these very sensitive horns.