How are your other invertibrates? Especially corals? If they are also apparently dying, i would strongly suspect copper poisoning; in fact, I'd still consider copper in your sand to be the prime suspect. This is assuming a few things: A) Many of your bristleworms are dying at the same time, after having been healthy in your tank for a long time. B) Your fish seem unaffected, and your water tests okay, especially with regards to salinity.

You should probably test for copper using the best kit available. (Salifert)

Carbon's probably a good idea. Use ALOT in a canister filter under some pressure.
15-20% is not a large water change IMO. Do much more than that if you find copper, only make sure you're not adding more copper with your make up water, and make sure you match salinity and ph carefully.

good luck, and welcome to Maast!