Unique problem - one channel silent under different variables

Bought a mint (looking) Cary slp 98 on eBay, connected to two reconditioned Hatachi HMA 7500 mosfet amps, work perfectly for years since reconditioning - biamping NHT 4 towers, bought new. 
When the preamp is turned on, a series of pops through the speakers, then Only the right channel plays. Switched tubes, didn't solve issue. Changed inputs from preamp to left and right amp and immediately both channels worked. Switched off, waited 5 minutes and still both channels working. Came back a few hours later, switched on again (having reversed the inputs to normal) and again same sequence: left channel silent and after switching inputs, both work and remain working until I switch off and come back when the units are cold.
I can understand that that there may be a resistor/capacitor that need to be hot before working, but why would switching input cables from left out to right input of amp and vice versa immediately fix it!
so every time I want to hear music I have to go thro this rigmarole! Why does switching cables fix the problem????
any solution will be greatly appreciated! I'm sure it will end up back at Cary but I wondered  if anyone (Almarg???) has an answer?  Does the popping on switching on, both channels pop, provide a clue?