Does the EXN soundbetter than the 851N? Yes. Does the 851N sound bad? Not at all.
Here's the difference. The 851n has a good soundstage with nice stereo separation. It has a smooth presentation top to bottom which makes it easy to listen to. Like I have the last 6 years. I use it as a pre-amp with balanced output into a PS Audio class D amps driving Dunlavy speakers augmented by dual SVS subs. My only complaint is that the volume steps are too coarse.
The EXN took its place and it has a wider and deeper sounstage. Instruments and voices are have a more believable presentation and space around them.
The volume has much finer steps. Yay! Also, the StreamMagic app has an EQ that is very useful and tastefully done! Great job.
I'm still using both streamers in separate systems and applaud Cambridge in their pursuit of great sound at reasonable prices!

I am an old audiophile so I will say right up front I do not use all the features on the EXN100 that I could or should be using. That said, I did originally own the CXN100 when my local hi-end audio store offered me a great trade and price on the EXN100. I figured why not and I was not expecting too much of an improvement. Oh boy, was I wrong! I use an Ethernet connection, and use an external DAC. The EXN100 is used as a source and is played through a preamp, so, again, I am not using all the features the EXN100 has to offer. But even using it as I am, it crushed the CXN100, itself a marvel! The EXN100 gives me a very solid bass foundation, tight and you can feel the bass. There is more clarity to the music, and a more "live" feel. I use mostly Qobuz through the EXN100 and I am very pleased at how easy the set-up was. Overall, I did not expect the difference between the CXN and the EXN to be all that great, but they are. If one can afford the price difference, I would recommend quite highly the EXN100. It is an excellent performer and a phenomenal value!

The EXN100 has brought music listening to a whole new level. Crisp detailed highs, wide soundstage, superb instrument isolation. When also using it as a preamp its even better. And to top things off its Roon Ready so no extra device needed. I primarily use this with Tidal via Roon. The EQ feature in Roon works wonderfully with this device as well. I've made several profiles depending on the genre/artist as well as the output being for speakers or headphones. The display is beautiful but I sit pretty far from it so I mostly rely on the Roon App or TV for cover art and lyrics.

Easy to set up with great sound. The EXN100 streamer is a big step up in sound quality over my old Pioneer N50. Well worth the cost.