It's like you said in your original post it is a pretty good DAC in the NAD. It's not that some of these DACs mentioned wouldn't sound different, not necessarily better, but you were asking about significant improvement. You already noticed how you couldn't tell any difference when you got the NAD from it and the other DAC. If some of the ones mentioned look intriguing I would make sure I could return them with a trial period. I've certainly been there and done that.