I tried removing all the power cables and power strips. I couldn’t hear anything.
If you have a lot of noise on your AC lines, or are running hardware that uses SMPS that does allows noise back onto the AC supply (95% of all cheap hardware that ships with an SMPS, especially wall-warts, and a distressing percentage of high-end hardware too), then a good power conditioner can be anywhere from making inaudible changes to being an absolute requirement.
“Good” in the above case is down to it a) conditioning the line in a manner that address any noise/line-issues you have and b) doing it to the point where there’s an audible change (usually a lowering of the noise floor/blackening of the background).
The power-strip really just needs to be able to pass enough current and not add noise of its own. Though if the power-strip isolates the sockets it provides to prevent cross-contamination (e.g. with SMPS plugged into it), that can be useful.
Power cables can make a small difference in an otherwise well sorted system. Electrically, once they’re passing enough current and not adding significant resistance, its really just down to skin-effect filtering - which isn’t much of an issue at anything around audio frequencies, and shielding so they don’t pickup RF/EM energy and add it to the line as noise.
It’s worth noting that power cables with larger diameter conductors will generally offer lower resistance than smaller ones, however the device they’re powering can still only draw the amount of current it actually needs. And if it’s internal wiring, or PCB traces, offer higher resistance than the power cable, then you’re not really gaining anything with fatter conductors on the external cables.
Outside of obvious, audible (or measurable, as long as you’re being sane about it) noise issues, you’re best of focusing on getting your primary components (source, DAC, amp, headphones and even interconnects) to the level you want them before you start spending any significant sum on power cables and power-strips.
another factor is the “object” receiving the AC power and it’s sensitivity to the power input. My laptop and dvd player show no impact from the conditioner, nor does my computer monitor, while my plasma tv does. When we were developing color prints, the color enlarger showed a significant improvement in consistency when using a line conditioner.