The gear in question is really top notch, and would definitely benefit from upgraded speaker cables. I don't want to open a can of worms here, but every single cable in that system could be upgraded with good gains made. When I did this, I immediately heard a better focus, clear imaging that I didn't know I was missing.
I hesitate to recommend anything specifically, because I don't know what the general sound signature is of either the amp or Usher speakers in question. I've got Audioquest Rocket 44, they are the most expensive speaker cable I've ever owned. Wireworld also makes some pretty effective speaker cables, I've got their Oasis 7 cables.
I would suggest that the OP needs at least a cable that is 12 gauge or larger for those Ushers, which go down quite low. Also, the benefits of a solid conductor design over stranded conductors really surprised me. Something employing copper that is more than just oxygen free, or even silver, will improve the sound.
Food for thought...
I hesitate to recommend anything specifically, because I don't know what the general sound signature is of either the amp or Usher speakers in question. I've got Audioquest Rocket 44, they are the most expensive speaker cable I've ever owned. Wireworld also makes some pretty effective speaker cables, I've got their Oasis 7 cables.
I would suggest that the OP needs at least a cable that is 12 gauge or larger for those Ushers, which go down quite low. Also, the benefits of a solid conductor design over stranded conductors really surprised me. Something employing copper that is more than just oxygen free, or even silver, will improve the sound.
Food for thought...