Oversampling: How and Why

[bluefuzz]

everytime I think I've "got" the theory, I find I'm woefully wrong - but I know if I can hear something.

Actually, this is where you are probably wrong. When dealing with the subtleties of different (over)sampling frequencies and/or bit depths it is extremely unlikely you can actually hear any difference. Especially not without rigorous controls. Much more likely you are just fooling yourself. We all do it.