I searched everything around the main power amp IC (tsk tsk .. fancy using an integrated amp in a quality product), and eventually came to the conclusion it was faulty. I'd replaced a few caps around the place, but nothing would explain why one channel was all over the place while the other was quiet.
So, I bought a new one from eBay. I had no confidence at all that it would do the job, but you have to accept the improbable, once you have eliminated all of the other possibilities. And it did.
(Picture from the eBay advert I bid on).