Did you try adjusting the nut itself? Unless the nut is cracked somewhere and that's what's buzzing, it shouldn't be a problem. I had a Streamer LX 5 with a brass just-a-nut and restrung it E-C and didn't have any issues. Just a quick adjustment to get the action set, and it was solid. I guess if it's still buzzing and adjusting it doesn't fix it, then I would recommend the brass just-a-nut (make sure you get the right size, so if your bass is a broadneck you need the appropriate nut). It's solid, I can verify it works with E-C tuning, and it looks great too.
2006 Warwick P-Nut II #21
2008 Warwick Streamer Stage II 4