Bright is not a word I would use to describe B&W speakers, I have sold lots of them and own a few pair, not what the OP has, the speaker is last in the chain of audio
it could be a system thing that just doesnt work, room issues, isolation issues etc
Trial and error is the only way. my take is if one cant get the sound they are looking for with basic cables like Gator cable....STOP right there. I have heard very expensive cables that just plain dont sound musical, not saying they cant
but start with the basics, try basic cables and different sources if you can and see where it goes.