Corby, When you use the term "powered down", do you mean switched off, using the rocker switch on the rear panel, or to "Standby" mode? If the latter, I can understand why that would damage the circuit; there are still high voltages in the chassis. It might even be unsafe to pull tubes in that condition. However, it's true that there are a plethora of ICs used to run all the automated features, and these can be damaged by careless operation. I know this, because I killed an IC myself while installing new coupling capacitors in my Janus Signature. (Well, I converted it to "Signature" with new caps.) This was due to my goof; one of the new caps was shorted to the chassis.
As for "noise", I am not sure what to think yet of the Janus. However, it can sound great. I only wish it had a "Mono" switch, for mono LPs.