i'm enjoying this good mystery Craig. I did a similar troubleshoot with analog scope/same iPad app on a Rotel preamp. Found the lost right channel at a cracked solder joint. 

Finally.............after what has seemed like a lifetime I have bottomed this out, 2 failed components TR52 and TR54 on the left channel. Two failed components caused me considerable confusion.....I suspect TR54 went down and in turn burned out TR52, I'm probably wrong there but not to worry. One thing I have learned is that replacing capacitors on a working machine and fault finding down to component level are two completely different things....this has been a war of attrition, and all for a machine I will be keeping for spare parts for the two healthy units I already use.........Well now I can get on with replacing the capacitors on Steve's (sounds heavenly) 4000 and also the same on the two beovox 3800 speakers. I will start a thread on the speakers exercise in due course...... ;¬)