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.