Power Cables on Subwoofers do Matter

^^^ So you believe whatever he said?

I will do better. I will test myself. 

I have a local Hi Fi store that has lot's of equipment including very expensive power cables and I will test them.

So, ... 

 I went to my local store to test a subwoofer and a dac. I know very well the salesman and we have a good relationship (he sometimes let me test at my home some equipment if they were returned / opened by a different buyer). 

After testing / speaking about DAC's I asked him about power cables and if he thinks they can make any difference especially for a subwoofer. 

They have lots of cables so It just happened I could test a Zavfino cable with a Monitor Audio subwoofer and an SVS. To my big surprise for both subwoofers it was a difference between the standard cable and Zavfino Fina. It was not a big difference but you could hear it definitely. I didn't expect that for sure and I was surprised by the outcome. 

Then he insisted me to test a more expensive cable: gigawatt powersync plus. He told me that out of all the cables they tried this was the best bang for the buck. Wow, this cable really made a difference: the bass was more tight, lower frequencies, deeper, even the stage seemed wider. Everything sounded so clear, like being a different subwoofer. I really can not understand how a cable can make such a difference but it did. He told me that there are even better cables but those are a few times more expensive and it would not be a bigger upgrade over this one. It appears they use it for demo with all their equipment so I ordered one because I didn't want to buy a used cable. 

When it arrives I will most likely test it with the dac I currently have and with the amplifier. I may end buying another one or two ...

So in conclusion: I guess I was wrong.