I spoke at length with Garth P. (the designer of the Niagara line) about this same issue before purchasing my own Niagra 5000.
Although he wasn't trying to simply push his company's products (Garth is NOT that guy), he was clear about designing the Niagara and the AQ power cables to be complementary. He intended the Niagara to be more than a power conditioner, more like the main component in an integrated ground-noise management system and a current reservoir.
For this reason, he strongly recommended combining Niagaras with Audioquest power cables that he designed, both on components plugged into the Niagara and for the Niagara itself. Supposedly, working together, they form a topology that shunts AC noise to ground for all components connected to what is essentially an AC-power network.
So I spent a few grand on mid-level AQ cables when I purchased my Niagara. Disclaimer: I had no chance to A-B the AQ upgrade with other power-conditioning alternatives. And b/c I had to upgrade cables and Niagara at the same time, I couldn't audition them independently. But at the risk of repeating a cliche, what I heard was so dramatic, even before any break-in, that I immediately lost interest in A-Bing. Sue me, but if you were there, I bet you would have agreed.
I'd consider this upgrade to be one of the top three most dramatic improvements in my system -- ever. There was an enormous expansion of soundstage with images spreading across the room in what seemed to be a more authentic d; imaging became more "physical", with images taking on a distinct size in space, surrounded by palpable air; complex arrangements (think ELP's "Closer to Believing") sorted themselves out so that for the first time I was able to easily hear that a background "buzz" on a familiar recording was actually a buried-in-the-mix chorus, not a string section; and, of course, tiny details like ambience/recording-location acoustics ("you are there" v. "they are here") & cymbal fade-outs became much clearer without sounding "etched"; etc. Pretty much what you'd expect by eliminating low-amplitude noise and distortion. Surprisingly, I didn't expressly hear a blacker background. But then again, I was so gobsmacked by the more obvious improvements that I didn't even listen for it.
I think Garth's design goals were to reduce what sounded a lot like what we used to call TIM or SID related to interactions between high-frequency/low-amplitude line noise and tiny details in a source signal. What I heard supported that hypothesis.
FWIW, at the time I was listening to Harbeths with Silver Apex cabling, and mid-level audiophile components, like Ortofon 2M Black cartridge on a George-Merrill bespoke turntable, Oppo BD-105, Class D Elac Alchemy amps -- nothing fancy. Nonetheless, the all-AQ upgrade was pretty dramatic, almost "shocking," even on this gear.
I think we all know about cables. Upgrading can be a crap shoot, since the interaction between a particular cable and a particular piece of electronics is complex & often unpredictable. So although there are plenty of great power cables out there, it made sense to combine this expensive power conditioner with cabling designed specifically to be used with it.
YMMV and I'm just reporting on my thought process at the time, what trusted sources told me, and what I heard. But today my significantly upgraded system is loaded with five-figure components and I still have no beefs with the AQ power components I bought pre-pandemic..
If you, like me, have no opportunity to compare multiple cables on your Niagara, at least consider the educated, but KISS, solution that worked so well for me, and that was recommended by a brilliant engineer with deeper experience with your conditioner than everyone on this thread (including me) put together.
