B&K amp popping with receiver pre outs

At my weekend place, I have a Yamaha rx-a3000 receiver which is acting as preamp for some vintage b&k amps. The really early generation b&k's (black with gold lettering) work fine and sound great. But, if I run the next generation, with the red lettering And musical staff on the logo, they all pop when I mute or change sources on the preamp. 2 techs found nothing wrong with the 2nd gen Amps. Any thoughts? I measured no dc offset in the pre outs. All the amps sound good though. Thanks.
My B&K EX-442 Sonata does the same.
PS the setup is used for home theater setup hence the multiple amps but the popping is heard on all sources.
Skriefal what preamp are you running? The b&k's don't pop with other preamps, but the yamaha is super convenient so i'd like to stick with it.
I'm using an Onkyo TX-NR3008 AVR.
well i dropped the pair off to be evaluated together. will advise hombres.
What were the results of the evaluation?