External speakers connected to iMac work intermittently

I have had an excellent external speaker system connected to my 4 year old 27" iMac since I have had it. Recently, these speakers stopped working after a day or 2 and the only way to get them working again was to shut down, disconnect the AC plug and start over. Then they stopped working completely even though I followed the above procedure multiple times. I concluded the speakers were shot and bought new ones. Before opening the box, I thought I would try 1 more time. Now they are working again! Any ideas?