The XR, Nu (and its predecessor, the Neo) are Extended Response Pickups utilizing low-impedance coils (Lo-Z 350Ω to 500Ω DCR) which inherently have full frequency response from 20Hz to 20kHz and beyond. Unlike traditional high-impedance passive pickups there are no big peaks and dips in the spectrum, typically perceived as the pickup’s “character”. That does not mean that Lo-Z pickups lack character. In fact a Lo-Z coil’s characteristically wide bandwidth gives it a voice all its own. The pickups are very rich in harmonics, with uber-clean sounds, lots of overtones and crystal clear highs. They have a lot more harmonic content than even the bright Stratocaster pickup (which has frequency response that rolls off at around 5KHz.) On the low end of the audio spectrum you get tight, well-defined and focused bass all the way down to 20Hz.
Guitars have their own individual sonic signature, thus the idea of a low-impedance pickup which imparts very little coloration to the guitar sound is appealing if you want to preserve that pure guitar sound. Starting with the unadulterated sound of the guitar, you have full control of what gets filtered. You have the power to achieve whatever tone you want, using appropriate EQ, rather than being limited to a fixed, predetermined guitar tone dictated by the pickup manufacturer. One can always shape the frequency response at any point in the signal chain using various analog or digital filters. Having a wide and flat frequency response gives you total freedom to sculpt the tone.
The use of low impedance pickups is not a new idea. An important example is Les Paul’s favorite guitar and the one he used since the early 1970s: the Les Paul Recording Model. For Les Paul, it’s the the ultimate recording guitar. It was designed to be plugged directly into a mixing console. (Now that’s a cool idea!) The only problem was that only a select few at the time could afford owning a mixing desk. In addition, the output of the low impedance pickups could not sufficiently drive guitar amplifiers like a Marshall (Yet, driving amplifiers into saturation was what people wanted!). Les Paul worked his whole life to get rid of distortion. It took him years to develop low impedance pickups with that bell-like clarity and direct to board recording capability. It did not catch on, perhaps because it was way ahead of its time.