If the system is working as it should and did, you may be experiencing a fresh listen "aha". You were away so long that your ears are no longer accustomed to the sound of the system, and you've been hearing other sounds in the world, and your system sounds different now. Otherwise, look to make sure nothing has gone bad in the system. 

You will have to rerun in your system for at least half the time of new .

My system , particularly my power amp sounds lifeless if there off for 12 hours or more . I never shut them off , just turn the volume down at night .

This begs the question , why would you take leave of your system for several years ? I can't imagine not hearing my system for a single day . When there was a low point in my life ( family tragedy ) , I relied on my system and music even more . It was always there , even when friends were not .

OK.  So you turned off your system.  Did you MOVE it? 

Hello?