I have several power conditioning options... several DIY Hammond 193L & 193M chokes used as parallel filters, a PS Audio Soloist in-wall filter, and most recently, a PS Audio P10.
When I first started using the NC400, I went back and forth between straight into the wall or in parallel with a 193L & a 193M. With the choke filters they were a bit smoother on the high-end, but slightly less dynamic. I left them into the wall at that time.
I added the Soloist in preparation for the P10. I tried the NC400 into it, heard a bit more reduction of dynamics with similar smoothing in the highs compared to the chokes, and again stuck to straight-in.
Once the P10 arrived, I liked it immediately... smoother highs, more detail & articulation across the spectrum, same or greater dynamics, less change from 'good' powerline times to 'bad' powerline times. I also tried the P10 into the Soloist (as PS recommends) and with the two Hammond chokes in parallel on the input. I didn't think the Soloist added much and still diminished dynamics (only slightly now). The Hammond chokes in front of the P10 added their bit of smoothness with no reduction in dynamics and they've stayed in. Still, the effect of each with the P10 is only a fraction of what it does and I could live happily with just the P10.
Yup, the P10 is expensive and heavy. But it was very worthwhile with the NC400 to my ears.
YMMV.
Greg in Mississippi