Before I packed Veils in recently, I was tuned G-C-G-C-F
I always found that tuning a bass to match a drop C guitar is kinda tricky when using a 5 string. I used to play drop Bb with Veils a while back and tuned my bass up half a step (before dropping) to match. Presently in San Sebastian I am tuned in Eb and I much prefer it. I have always thought with metal that if you are simply tuning down to make your songs heavy, then the songs aren't good enough on their own (think Lamb of God's first couple of albums). If you have other reasons for tuning down then go for it, but I find that you tend to grow bored of it after a while. You lose clarity the lower you go and, to me, it just isn't as comfortable to play!