meron pa din thud sound even if volume is at zero level. sa ibang amps ko naman upon power on meron thud.
the tubes have nothing to do with that,
the power amp is a solid state and connected to speakers via relays,
on power down, this relay should turn off quickly,
but on power on there must be a delay of a few seconds,
this the heart of your problem,
ss amps do have turn on/of thuds and so uses a speaker
time delay relay, this is very rare in pure tube power amps...