Even on max, there is very very little hum. Not dead silent, but at this volume my ears would bleed with music playing so I think it's totally irrelevant. At normal listening level, the 332S background is completely black to me, meaning it is way below ambient (late-night) noises from appliances (refridgerators, pc's etc) elsewhere in the house.
Did you check the hum level with your input source disconnected? Hum can come from anywhere (ground loops, source...). In my system, the noise and hum bottleneck is now clearly not the 332S, but the Valab DAC that is connected to it (and even that level is absolutely not affecting my listening experience). If the hum is still there without a source connected, I'd try some tube rolling+burnin before drawing conclusions. I'm very happy with my revised and silent 332S, hope you will be too!