A slightly different observation. I too have the Noise Destroyer and have gone back and forth from liking it to thinking it may suck some air out of my system.
My conclusion as follows:
When I just leave the Noise Destroyer in my system, (plugged into the wall), I find music is wonderfully grain free with a huge sound field with lots of air. When I do rapid A-B comparisons I find it sucks some of the air out of the outer reaches of my sound stage along with any grain or hash. Conversely when I listen to my system without the Noise Destroyer for a few days I find some recordings have some hash in the upper registers but I do notice more air.
I think this unit takes time to settle into a system and then if you feel your system sounds dark then that is the time to take it out. Factors that could contribute to this effect are- dark sounding electronics, speakers, and cables. If you have boxes on all your cables and warm electronics then the Noise Destroyer may not be your cup of tea. If on the other hand you do not use a line conditioner, have open and transparent sounding electronics, and transparent "sounding" cables then the Noise Destroyer may be all the line conditioner you'll ever need. With or without the Noise Destroyer my systems sound is holographic in the extreme with air and special cues that are holographic. As with everything else it boils down to system compatibility and personal preference.