Most people have no idea how important the power supply is.
A quality power supply removes all the nasties off the line and turns AC into very clean DC.
If you need some magical mystery power cord to "help" a device, then it probably has a crap power supply. That goes no matter what the MSRP of the device is - even some kilobuck devices have awful power supplies.
I can't comment on dedicated transports, but over time, I've owned CDPs from Copland, Krell, Nagra, and preamps from Naim, Audio Reasearch, and also various Stax Energisers. All very different designs and all from respected manufacturers that have stood the test of time. And every single one of them has benefitted from better power cables - my considered subjective opinion of course.
In my current setup of Nagra CDP and Stax Energiser, I have an Audience Power Conditioner on a month's trial (expect a gushing review to be posted later. Broadly, a competitor to the Furman). I was taken aback by the improvement this seemingly simple capacitor-based conditioner has made to both components. The science of this doesn't make any sense, but the sound improvement is beyond any doubt.
Sticking just with Nagra, this company has a long standing high reputation amongst professionals for their studio equipment, and is not known for their frivolity or lack of attention to detail. I for one will not be visiting their design centre in Switzerland to tell them that they produce crap power supplies.