I don't think the OP is confused about hiss vs hum, but maybe a little confused as to the distinction between noise and distortion.
Is your hum through the speakers or from the transformers of the PSUs in the components themselves?
In terms of the 'transformers' (actually they're probably SMPSs) for low voltage lighting, then these won't have any effect when the lights are off (no current = no power = no EMI), also the main house lighting will be on a completely different circuit to the mains power distribution making that a less likely source of interference.
For the microwave, there should be an isolation switch somewhere in the kitchen, try turning it off using that and see if the hum goes (if not it's something else - of course it could always be coming in on the mains supply from outside your house).
Huge, I have a very distant soft hum in the loudspeakers of my home office. I think this is some mains wire being close to a cable, or a smps. It is unforuntatly a bit cluttered again in my hifi stack. Need to clean up things: all office hifi, printer, raspberry pi, my synthesizer, switch and pc are on the same power bar. I think the issue will go.
My SN1 transformer loves to hum pretty loud. I took it to my parents house and there the hum was pretty civilized. It was there, but much less than in my house. I've noticed that when I print a document the hum gets more intense - the Samsung printer seems to send a lot of noise on the mains. My microwave (from 1993) is giving the same hum as my SN1 and it is one of the few devices I can't disconnect. It is in-build in the kitchen. You are probably right about the lights. There is a dimming switch and there is indeed no audible hum or so.
By the way, I have an old house. My electricity is really a chaos. Livingroom, Kitchen, Home office and bathroom are all on the same ring. It is quite bizar but takes significant money to change. I'm actually impressed by how good the Hifi still performs. However I have earth wires, I don't think the earthing is operating properly. Several metal devices like my macbook or telly give 'resistance' when I stroke them.