Direct hands-on comparison:
My system includes a pair of 220wpc amps driving an easy load (Harbeths). It always sounded OK, but never quite like what was promised by the most credible published reviews. Cable upgrades, room treatments, vibration mitigators, all made some difference, but I felt there should be more.
After extensive research, I added an Audioquest Niagara 5000 conditioner and full set of complementary AQ power cords. Invested about $7000 total. The transformation was dramatic. Holographic soundstage, more realistic imaging, and vastly improved reproduction of tiny details, such as room ambiance. Overall, one of the biggest sonic improvements I’ve experienced in any system I’ve ever owned.
And oh yeah, the addition of the conditioner also improved headroom. Yup, that’s right. Maybe it was just a matter of better transient response making everything sound punchier, but the improvement was dramatic..
The ponit is that every setup is different and a successful power-conditioning strategy calls for the research required to make an educated decision. In my case, I had a 7-year-old house with combined utility power & rooftop PV. No ground hum or aluminum wiring, but during the day, a lot of inverter noise. I blindly tried a few $3-500 power conditioners that were little more than surge protectors, and heard no difference. It wasn’t until I took the time to understand the technology that I had such profound results. I’d never consider running my system today without the AudioQuest power-conditioning in place. I’m sure that analogous Shunyata gear would also provide significant improvements.
Properly designed conditioners, like the sophisticated Niagara line do a lot more than reduce audible noise. One benefit of the Niagaras is that they effectively create reservoirs of power that provide the energy needed by an amplifier to reproduce peaks. Randomly matching a crappy $500 conditioner with a beefy, current-hungry amplifier can certainly limit dynamics. But if you take the time to do it right, you stand a good chance of greatly improving your sonics.
And the quality of your amplifiers doesn’t necessarily obviate the need for power conditioning. Michael Fremer himself still uses a Niagara front end for his Dartzeels (!), even after having his entire house wiring, all the way to the curb, replaced a few years ago. Even running a dedicated line to your system can be effective, but in some cases, you need more.
Bottom line is that every system is different and every owner has to decide what type of power conditioning (or amended house wiring) is most cost-effective in his or her specific case. You can’t make an across-the-board statement one way or the other, even re:parameters like headroom, w/o first spending time on self-education and analysis on the order of the effort you’d make when shopping for speakers.
I speak from personal experience.