@jazzman7 Thanks for the comments. I am not discounting your suggestion of trying different interconnects. At the moment it's a Chord C-Digital coaxial and a cheap Toslink. I have tried the Chord cable vs a cheapo one and heard zero difference, which only increased my scepticism about cables. But I am trying to keep an open mind. I just want to think it through before spending on interconnects.
I agree about 'NADness' from the Node potentially colouring everything downstream. After all, when I use UAPP on USB from an Android phone into the Qutest and other dacs I own, I can clearly hear the Qutest is far better, almost holographic, etc. UAPP is definitely bit perfect digital output. Not so sure about the Node. Bluesound support say the Node's output is bit perfect even with variable volume, which raises an eyebrow.
I might use UAPP on USB out of Android for serious listening, and use the Node 2i for convenience when I'm wandering around.
But that doesn't explain why you in your system with your ears can clearly hear the difference when feeding the Qutest from the Node. Maybe your system is better. I've got a good amp and speakers and fairly experienced ears though, so I'd expect them to be resolving enough.
The mystery continues ... I hear you about interconnects, but want to explore a bit more before splashing out on them.