I have heard the entire range, including the new top model that is about $156k. They all share the same basic sound—relaxed, natural, and easy to listen to for many hours without being distracted (you engage fully with the music), smooth (but not dulled in impact or murky in get that smooth sound).
All the models basically have the same digital conversion circuit which is minimalist and use vintage chips chosen for their sound. The model differences are primarily in the analogue stages and they involve progressively more expensive parts, wire, and more elaborate circuits using their very expensive transformers and chokes.
For their performance, I don’t think the lower level stuff is expensive, particularly because the Audio Note sound is sort of unique. But, the price does rise very steeply beyond the DAC 3 level. My own personal preference and willingness to pay would stop at the DAC 4 level (I have too much already invested in a Naim 555 server/power supply/UnitiCore server to convert).