Ever since I have had my Mono 15 tube amp, I have noticed this sort of "fwit" sound it makes when I turn it off. I do turn the volume all the way down before switching off. I assumed it was just something tube amps did - by my Nephew commented that HIS tube amp (a hot rod deluxe) does not make a sound like that when he shuts down. Anybody able to shed light on this strange sound - and, more importantly, do I need to worry about it? Thanks!