Another update:
Well, this has got me stumped!
All was OK on the scope.
Into the main system again (didn't bother with setting up a headphone test). DAC into attenuator into pre into 2 x D290P (in mono).
One amp worked just fine (let's call it the "good" amp). The other, when connected causes trouble! This "bad" amp actually works just fine with all my other pre amps, so not anaccurate description, but it'll do for now.
I tried good amp on both left and right out on the pre = all fine. Therefore source is good on both channels and there was hardly any pop on switching.
Tried reconfiguring good amp into stereo, and on connecting both pre-out = trouble.
Tried connecting de-powered bad amp onto pre (without good amp connected) = trouble.
This "bad" amp sets off the pre for some reason. The "good" amp is fine on both channels of the pre. I tried both of the two different mono inputs on the bad amp, but I can't see how this could make any difference.
I didn't short the inputs when I was measuring DC offset, so I'll do this again. Likewise, I was thinking about dropping the nulling circuit into the board and seeing if that helps with anything.
I'll recheck my soldering, but can't see how this would make it work on one amp and not the other. Both D290Ps are similar age and have both been fully re-capped recently and run LME49710 op amps in the first stage.
Curiouser and curiouser...
Jon