Marantz AC 500U

From 5 amplifiers only 2 are going to protection mode, 3 other are OK. However, since the 1-st amp starts clicking (protection mode) I had to reduce volume on the preamp from 12 to 9. Then it is not clicking but sound is not loud. I got another amp and conected to this preamp and it did not change anything. I also replaced 2 Niles volume controls for 2 rooms connected to this amp (Sonance SONAMP 260x3). It looks to me that Marantz preamp provides now different output for the amp, maybe to strong so that amp goes to prevent when I keep volume on the preamp in the same position which I had for 12 years without any problem. I used entire system for 12 years, but only once every 2 weeks or once a month for 5 hours and on 40% volume on volume controls. I think that recently something happended to the preamp that also damaged 2 volume controls. I stoped having a sound on 1 pair of speakers and had to replace volume controls on the wall. I understand that amp will go to prevent mode to safe amp, but that clicking could damage the Niles volume control. When amp goes to prevent mode it does to safe amp, but not the volume controls on walls or speakers. THAT'S MY UNDERSTANDING. AM I CORRECT RE THOUGHTS??? I have Ph.D in mechanical engineering but do not know much about audio equipment. Now I am testing the system with 1 amp contected to preamp, I disconected 4 other amps to investigate my problem. Am I doing right thing and and again am I correct re my thoughts what's wrong with my system??? I know (from manual) that preamp should provide on output 1V/600 ohm, could these parameters change with time or for any other reason??? I would appreciate your advise...Andrew