Are upgraded power cables for subwoofers worth it?

My experience might help your decision.  My main system is pretty high end and incorporates a pair os REL SX 212 subs.  When I bought these 4 years ago, I followed REL's advice and used their power cables.  At that time there were other priorities.  About a year ago, I finally got around to trying after market power cords.  Audio Sensibility Statement is what I use throughout my system for interconnects and speaker cables, so that is what I ordered.  The decision is made easier because the company offers a return privilege.  Long story short there was a noticeable improvement.  So what can you learn from this?  First that improvement is definitely possible, and second, perhaps it might be something that should be considered as not a high priority upgrade.  I certainly have no regret for having postponed addressing it. 

Upon further reflection, I've been speaking specifically to my recent experience with the REL subs. In consideration many subs use class D amplification I'll agree they less sensitive to PC vs at least some REL's. Class D much more efficient, switching power supplies makes them less sensitive, yet I still heard slight differences with my class D subs, low level IC's made a larger difference here.

REL's implementing class A/B amps are another animal altogether, this same topology used in amps for main speakers, pc's make great difference with these amps. Now with subs dealing with much narrower range of frequencies,  voicing isn't the main issue, dynamics and transients remain. @billstevenson post correlates with my own experience with REL's.

My speakers have powered woofers and I use a pair of Shunyata power cords into a Shunyata Hydra 4 plugged into a dedicated Shunyata wall outlet. Not sure if there is that great a difference but I had all of these things hanging around so why not use it?  When I asked Sandy Gross about it he said that if he already had those cables and PC at home, he would definitely use them on the speakers.  

@sns I’m well acquainted with placebo effect, been around the block a time or two over the decades....How one can conclude PC don’t affect dynamics which I could also define as current delivery, just as it does with pre’s, amps, sources is illogical to my way of thinking. I suppose some just aren’t as sensitive to these things as others.

I wasn’t suggesting you didn’t know about the placebo effect! lol. As you mention, a lot of factors are at work bringing your subs together (your special design PC, a 20 amp 10 AWG circuit, placement, crossover settings, high-level REL connectors.) I will take you at your word that the PC’s you’ve designed are playing a significant role among those ingredients. (In other words, if you swapped them out with all those other things in place, the quality would suffer.)

In my case, after doing a bunch of work, I have not had any persistent problems with placement, crossover settings, or other things. My subs, with all those pieces in place, are seamlessly integrated and tight. They measure well. When I experimented with power cords, there was no improvement. So, for me, the argument of "no difference" from a PC is based on personal experience, plus REL saying there’s no reason they can see for it to make a difference in theory nor in their trials.

P.S. I just want to add that, of course, in all cases the cord has to be large enough to meet the current demands of the device, and that a sub will tend to draw more current. That's why, to my understanding a 12ga cord makes sense. A 10ga can be used, but with a 20 amp circuit, the wall wire is 12ga. Still, 10ga won't hurt though it will cost more.

I have a Martin Logan BalancedForce 210 which has a 850-watt class-D switching amplifier. I was feeding it with a power cable from Signal Cable, when I read a reviewer on PosFeed dicuss what an improvement an aftermarket power cable made to his sub, in the bass, but also in the midrange.  I value good midrange and have ML CLXs, which are pretty revealing.  So I bought a Sunyata Alpha NR for the sub (I have multiple Alphas and Sigmas in my system).  Shockingly, it had a dramatic effect on cleaning up my midrange, and the impact on the bass was positive, but not to the same extent.  I hypothesize that this is happening from the NR power cord suppressing any "back flow" of switching noise from the class-D sub amp from getting back into my system's "grid" and hence the other components.  But, who knows?  I just know that I would not go back.

Then I decided to upgrade my cables from my ARC REFSE to the sub from average grade to sub specific (and balanced).  I was shocked again ... they hardly made a difference.  Not sure they made any improvement.

I am a n of 1, but that's my data point ...

-docknow