So this may not be a fair "apples to apples" comparison exactly, as the DJM-900 is really the mixer that fits the category, but here goes!
Pioneer released the Nexus 2000 some years ago, circa 2011, so they are due to hit the world with another mega release sometime soon (I hope).
Let's break down the mixer in the familiar review format:
1. BUILD - it's Pioneer. Solid. Done.
2. Fades - Not MAGVEL, but they'll do the job (unless you're a scratch junkie, then maybe look elsewhere for a thinner mixer with a newer fader).
3. Sound/connectivity - again, Pioneer. Solid. Adds ability to run four CDJs via pro-link, no ethernet hub needed. Also has MIDI on/off, and MIDI START. Sick.
4. Layout - here is where this mixer gets interesting. Watch this video to get an idea of this big section in the middle, and the layout in general.
They present this four channel beast in a specific way on this video because the single factor that separates this mixer from any other four channel offerings in this broad digital-mixer category is that kaoss-pad-esque touchscreen with built-in sampler. The fx available on the screen are fairly fun to play with, but not game-changers. The "mix" mode on the screen is also fun to play with, also not a game-changer. It's just a 7-channel EQ. It has a place, but will likely only be rarely used in my world.
The SAMPLER is the thing that sets this mixer apart. You can record four or eight beats into the sampler, from any source you choose, and either play that sampled audio back on the same fader (channel) from which you recorded it, or you can send that new sample to a different channel. This allows you to layer a vocal sample over a different part of the same vocal track in realtime. You have the ability to then "remix" that sample via the touch screen, and you can record that remix if you choose. Think "serato flip" if that helps. It's really the same end effect - recording your "chop and slice" of a sample as data and looping it back live.
Its really quite cool. Pioneer clearly made the choice to keep the CDJ's creativity to a minimum thus far - which is why they have never appealed to me. To get around this, the Nexus mixer has the creativity built-in. I have the SC5000 players, which are immensely creative and fun - and I have the SP16 which allows for slicing and sequencing of samples. Since I sold the DJS-1000, I lost live sampling mode until June 2018. This mixer allows for one or two bars of live sampling mode, which does fill a void for some users (maybe even myself).
It is a GIANT mixer. The question is: are the additional processes in the center worth the extra 3 inches of width?
Well that's a personal question. Size does matter!
Here are my criticisms, bearing in mind that this mixer is from 2011, and my X1800 is from 2017.
The Nexus 2000 lacks dedicated filter knobs. It relies on the sweep/color fx section to the left for all features, so to get a LPF you must select that feature, and then twist. Same for HPF. The Denon (and the newer mixers overall) have a separate LPF/HPF combo knob AND the color/sweep fx. This allows for things like Dub Echo, which the Nexus 2000 lacks. Again, it's a different era.
The live sampler works with any signal incoming, timing is good. HOWEVER, the BPM engine for the bottom half is separate from the BPM engine up top. So, you can let the mixer determine the BPM automatically for the main fx engine up top, which it does superbly, but that info is not sent to the sampler. You must select "manual tap" and then tap the tempo. It picks up the tempo very quickly (and accurately) with just a few taps, but they did leave the two units apart. I suspect this is to encourage users to get the CDJ units, which send tempo data over ethernet to the sampler AND allow the sample to be sent back to the CDJ instantly for scratching.
Does that make the CDJ worth the investment? Not to me. I prefer dual layer and the mega-performance pads on my Denon Prime SC5000 players. I have no trouble tapping tempo once in awhile.
I have a long post about the Denon mixer on this blog already. Here is my short note:
The X1800 is more modern. Dual Usb sound cards (Pio has one), dedicated filter knob + fx knob, lights on each channel to show which channel has fx routing applied to it, the little touch-strip to change beat grids for fx, addition of dub echo and a few more modern effects. The X1800 also has dual microphone inputs - also found on every other 2018 digital mixer I'd ever consider owning.
Still, the effects on the Pio sound better (to me), keep tempo easier (auto-detect on Pio just works), and I can live without the "light guide" features that the Denon integration provides.
I have both in my hands at the moment. The next test - see if the Pio sends "MIDI START" as the unit indicates! The Denon still fails to do so.
If Denon would release a firmware update to improve the effects unit, and the detection system for BPM - to apply to the effects) I think I would stick with the X1800. The sampler is great and I'll find a way to use it in my set. It's frustrating that the Prime mixer is "so close" but not 100% there yet, for me anyways.
If you compare the DJM900NXS2 alongside the X1800, the difference to me is in the effects/BPM section - and that alone would be worth the swap (given the current builds). Even though the Denon sends "active track status" and the engine connect sends tempo, I'd trade those things for the DJM900's proven build, effects, tempo detection, and such.