Best speaker cables for under 500$ (Euros)

Yeah, a lot of guys feel that way. The problem is there are so many to choose from, and instead of relying on listener impressions most guys buy into marketing narratives. Anyway, if you would listen to Synergistic Research after a while it will become clear wires are just as important and contribute just as much to system performance as anything else. You can simply search around for the newest/best deal on used SR speaker cables. 

Some of the best deals out there will be cables made over the years when SR used Active Shielding. That's what I have, CTS, which are very affordable now being as they are over 15 years old now, yet still incredible performance being as they were the best speaker cables made back then. https://systems.audiogon.com/systems/8367 If you do go with SR Active the power modules offer a great upgrade path. The cables are great as stock, but for almost nothing can be upgraded with better caps and diodes that improve performance like you will not believe. Do this and it will be the end of your thinking cables are any less important than other components. 

Buy anything else and all bets are off. 

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...

Anti-Cable 3 might be OK . British Chord might be best with Euro .