pop as pre input is selected and after mute delay

I recently acquired a Nagra PL-L. It is a wonderful preamp. However, I seem to be experiencing exactly what Kal Rubinson reported in his excellent Stereophile review: there is a transient pop as I move the input selector switch from the off position to any other position, and then there is another pop after the pre-amp's 20 second muting delay ends. This happens whether the muting switch on the front panel is enabled or not. All of the tubes are properly seated, and the amp functions properly in every other way (and sounds extraordinary).

Kal found that both his first unit from Nagra did this, and a second "fully run in" unit that they sent him did exactly the same thing, so it must be endemic to the preamp.

So my questions are: (1) what is causing this? and (2) does anyone have any ideas on how to avoid it? [I do not want to switch my power amps on and off to avoid the problem, because they are Kharma MP150s which sound much better left on all of the time] (3) how harmful is a transient pop of this type to my components downstream?

A couple of notes: I have written Nagra, and I am disappointed to say that they have not responded. Also, the preamp is dead quiet with virtually no background noise or hiss. Finally, these pops, while unsettling are not terribly loud.

Thanks as always for your help