"Pop" in pre-amp

I recently purchased a used B&K Pro-10 pre-amp on-line. Whenever I turn it on, there is a "pop" in the speakers. This also happens if I switch in or out of the direct bypass mode. Does anyone have any idea what it might be? Is there a quick fix? Or do I need to take it in to be serviced or repaired? Thanks.
Rule 1!!! Switch pre amps on and off ONLY with your amps off. It does sound a little unusual for a solid state unit to diplay start up transients. Until my recent change to tubed pre amp I never really felt the need to observe this rule. Maybe another Pro-10 owner can shed further light?
I agree, the power amp is last on, first off.
Since this is ss gear, why wouldn't you just leave everything on all the time? No pop. Better sound.
Must agree with Jim. Leave it on = always warm; it will sound better that way & no transients to cause problems.