I have HSU VTF3 MK3.
When the subwoofer is on there is a hum that is noticeable with the gain partially up (say around 8 oclock). If I turn the gain up the hum gets louder and is very noticeable at about 12 oclock and plainly audible throughout the room at max. So it is not a huge buzz but definitely annoying to someone actively listening, and concerning since I'd like the capability of turning up the gain for future use in a larger room.
Now the kicker here is that with only the subwoofer AC cord plugged in I have no hum at all gain levels, with any RCA cord plugged in I hear the hum, even when the amplifier end of the RCA is disconnected completely from my AVR. (i've tried RG6 with f-rca adapters, quad shielded RCA from monoprice, and random RCA cables around my house). RCA in Subwoofer with nothing else connected, hum. RCA out, No hum. IT is a constant hum, not hissy or static-like, and it does appear to be mitigated as I turn down/activate the Low Pass filter.
Also, the hum dissipates slightly when plugged into the avr. One other weird thing, when I touch the outside of the RCA plug (not the middle pin but the metal termination outside ring), the hum goes away completely. What is this hum, keeping in mind the HSU subwoofer has a cheater plug built into it.