Get an isolation transformer and a cheap storage oscilloscope.
I'm sure for someone experienced the benefit is marginal but having a bunch of probes and being able to see the waveform feels like cheating for an amateur like me.
They're not that bad. You can get a good entry-level oscilloscope from a company like Rigol or Siglent for ~$300.
A good, fast reacting multimeter is likely better thing to get than a cheap scope.
That said the cheapies might be usable for debugging audio stuff.
I am also very much in the belief system that you should not buy an expensive tool before you are frustrated with a cheap one, most of the time people get into random hobbies and bow out six months later with way too much crap on their hands, and often time tools are more for show than for real world use.