New power cord needed for speaker with powered subwoofer?

Yep.  Like Nonoise above, every single component I have tried has been improved with the use of a suitable aftermarket power cord.  Not subtly, either.

My experience with Vandersteen Quatro's was that some Elrod Statements feeding their built in subwoofers brought worthwhile improvements in bass reach and tonality, and also brought better nuance and tonality well up into the midrange.  That surprised me.  The Quatro's come with a very heavy gauge OEM power cord, unlike many, but the aftermarket cords were definitely worth it for me.

@jbrrp1 Is it possible to boil it down to materials, gauge and manufacturing technique / innovations?.  The price for cables is all over the place. Just wondering if you can help. Thanks

I'm sure the cable that came with your speakers is just fine.
Before you go swallowing audiophile lore about cables and (likely) spending unnecessary money, consider:

The concept seems utterly ridiculous on it's face:  this idea that a manufacturer, especially a high end manufacturer, would obviously cripple his own product, merely by not choosing an adequate power cord.  I mean, are we talking about competent design...or not?  Why would you buy a speaker from a designer who was not competent enough to even choose a friggin' power cable with the right gauge or electrical properties for the job????

But, it's your money. In the land of the audiophile, anything goes....

Use the cord that came with the speakers unless they gave you one from a 1970 hoover vacuum cleaner. I agree why in the world does anyone think that the manufacturers of these expensive componets think they  turn into skinflints when it comes to the power cord. If they are that cheap then don't provide any cord and advise customers to buy aftermarket. 

@prof @djones51 But how does one reason about it, like maximum current drawn, material, gauge, length and contacts etc of the cable.  

There is some evidence of a good cable (mostly better AWG, for speaker and length etc) to make a difference(may be worth it or not is a personal matter) : https://www.lifewire.com/speaker-cables-make-a-difference-3134902

Something about power cables: https://www.audioholics.com/audio-video-cables/power-cables, written with help of blue jeans so not sure if it is completely impartial. 

I seem to think that good construction(preventing hazards) , proper gauge for the application, length, material and rating are to be considered. I have seem some manufactures do not even mention the gauge and list for a lot of $.  I would stay away from them.

Once we have those objective factors it will be easy to see what one gets for the $.