Some tube amps even without noise pot have no his or hum, it depends on details of the builder's assembly, such as routing of parts and distance between parts and top table.
So the top table should be aluminum that is not magnetic and still absorbs the noises.
I don't want to drag this off topic; I'll post once only to make a correction/qualification.
You are correct regarding hiss and hum, but with qualification: hum will be audible to a degree if running directly heated tubes with AC on the filaments and efficient speakers. For example, a quality 2A3 SET build running good tubes and AC filaments will have 2.5 to 3.5mV AC at the binding posts. I've gotten as little as ~1.5mV from a 2A3, but hum was still audible very close to the speaker. PP with AC filaments can get lower, due to cancellation effects. DC filaments, if well-implemented, should result in an essentially silent system.
Aluminium is an inexpensive material that is light, is easily worked and has okay-ish shielding properties for audio; exposed aluminium oxidises, making electrical bonds (earthing, 0v signal grounding) problematic. Steel is a better shield of audio-frequency EM radiation, for example from audio transformers. Steel does not shield from high frequency RFI (in the GHz range) like aluminium, but that is not an issue in domestic tube audio circuits. The Austenitic Stainless Steels are nearly ideal: common 302 grade is essentially non-magnetic, has good shielding properties for audio, is strong, does not oxidise under domestic conditions, can finish nicely, is not crazy expensive... but can require specialised skill/tools to work.
Cheers,
Ray