Your old sub had a fault. Filtering isn’t about removing hum, it’s about removing interference, switching noises, stuff like that. If you don’t hear such things you have nothing that needs to be cured. It’s not about being open minded, it’s about knowledge. If you have an amp that clicks and pops all the time I would try substituting that before trying anything else.Mains conditioners.
British Telecoms use mains conditioners to reduce mains interference. In fact they manufacture their own. None such model is the BT MCU5A/125.
Just saying that if mains does not impact sensitive electronic equipment they would not bother as it’s nothing but a cost to them.
A Hifi is nothing if it’s not a sensitive piece of electrical engineering. Yes they should be able to handle noise on the mains, but there is a limit to everything. Not all Hifi is made equal and we don’t all have clean mains rings. My old subwoofer in my previous house used to pick up the 60hz mains hum. It was subtle but there and quite common. Maybe a mains conditioner would have helped , who knows. I don’t as I didn’t bother trying, but I’m going to keep my mind open to the idea.
With that said I would Only entertain the idea if It was cheap.