Strange noise after amp's off ???

Most-likely it's the way it is. I've never used Plinius 8100 but I assume that Plinius powers down rapidly with no delay.
I suggest using a power conditioner that will turn down your amp slow and you will hot hear any "dong" on your speakers. You will have to keep a switch on your Plinius always on.

Turn the pre off first.-?

Hi,

I've asked the dealer, and he said that it is normal...
Anyway I'll try to get a power conditioner...

Any suggestions for power conditioner under $250 used/new???
Thanks,

A line conditioner won't resolve this but the turn-off noise is a non-issue. Switching off power at the conditioner isn't any different than at the amp, but the electronics should be left always on anyway. Lots of threads on this subject are available in the forum archives. A PS Audio unit might exhibit a slow decay shutdown, but I dunno that for a fact & beside that they are expensive & inefficient.

Have you looked to see if there's anybody in there? Safety first!

Sincerely, I remain