pop with preamp outs Yamaha rxa3000 and vintage b&k amps

I'm running the pre outs of a yamaha rxa3000 into several b&k vintage amps. With the real first generation b&k's with gold lettering, there are no issues and it sounds awesome. With the blue/red logo era b&k's (the ones with the musical staff on the logo), I get a pop whenever I mute or change sources from the yammie. They otherwise sound great as does the whole system. I had one of the later generation b&k's evaluated and there were no issues found by elite audio in buffalo. any ideas? Thanks.

bumpski!