My Levinson ML-26 preamp has no power button. When it's plugged in or unplugged, it releases horrid noise way worse than a thump. I was always careful but we had some sort of power glitch and the preamp went off while the power amp still had some current, and it blew a tweeter in my Magnepans.
The problem was the power glitch, but actually a malfunction of a Monster power conditioner I was using. That sucker was retired quickly.
None of my other preamps do this. My Proceed power amp thumps when I turn it off, but only when connected to my BAT tube preamp. Otherwise quite as can be.