popping sound in sub when input selector is switched

i have a pioneer vxs-47 reciever and had a popping sound in sub when reciever would power down,had repaired under warranty,and when hooked back up, i now have a popping when switching inputs,any idea, i also have failed messege when using macc function,i,m not sure repair center if familure with this reciever,pioneer had them replace the dsp whatever board,it took over a month to repair,and i have to drive 70 miles for service,not happy,any ideas whats happening with my reciever,thanks,