Think before you knock.
I busk with panpipes or guitar. The pipes don't need amplification, but I play a lot of classical, which is not written for solo; hence a small amp with backings - orchestra/church organ/my own guitar.
I keep it down to avoid getting moved or disturbing other buskers.
Classical guitar and songs with finger-picking just CANNOT be heard without the amp - "projection" alone won't cut it I'm afraid!