JohnK is correct. The OP not damaging his equipment should consider himself lucky. He ran his system like this for 10 years and might be lucky enough to run it for another 10 years but the fact is that even if there is only 1 amp ON at the time, if they both share the same connection, there will be some leakage current going to the amp that is off. It's like feeding an amp through its output with very low current, never a good idea and susceptible to damage.

Bruno
M80s/VP180/QS8s/EP600/AVR-890
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"The problem is choice..."