I am a big fan of feedback, undertones, and overtones. I can get that stuff out of my amps at relatively low volume with my tube distortion pedals, and from my now gone Pod XT which gave wonderful feedbacks etc. even at whisper volumes.
However I remember the days I had my hundred watt Marshall Plexi. I got the most amazing feedback, undertones, and overtones from that thing when turned way up. I had marks on the floor that I could get certain tones from the direction I was standing, and if I moved from that spot it turned into something else as interesting. But the sound was Earth shattering. I am amazed I can still hear well. I do have a slight tinnitus ringing in my ears but over time I got used to it....