hum is always a perplexing issue, and there are ways to eliminate them..
in the Dusit show, i always bring with me an isolation and line conditioner to power up our gears..that helps...
but in home settings, it is better to trace the source of noise and do the filtering right at the source...
going back to tube power amps with OPT's, i make is a point to add a zobel network at the speaker jacks, 10 ohm 3 watts in series with 0.01 ufd/630v film caps to avoid the problem of running the amp at volume with no speakers connected...