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HELP!! Mitsubishi DA receivers

I have 2 Mitsubishi DA receivers, one is a DA-R11, the other is a DA-R17. The R11 quit working on both left and right channels because of bad speakers(?) Have new good speakers and still won't work. Should the protection circuit reset itself? The R17 has crackling sound on the right channel. The speakers have 6 ohms impedence each and the receivers require minimum of 4 ohms. I have had the DA-R11 since 1982 and have had no problems hooking up several different speakers over the years. What is going on?