Well... your rite about hi gain yes, and I can sustain a note clearly and at a high sound quality fr over 10 seconds easy. But thats probably cos I have a really good amp n guitar.
But, if its a trebly sound, particulary if my wah it switched on, then i can just stand in an direction of distance and pick the note, it will then shreak and hold for infinity!
Actually the closer I go to the amp the harder it is too get feedback because they notes jus phase out and go to mush!