I touched on this subject a while ago. After trying E-standard to B-standard and everything in between, my band settled on drop C# as our tuning. It fit our singer's vocal range, and sounded heavy without having to do much screwing around with EQ and effects. The big concern for me was what strings to use.
I got into the mindset of using balanced tension sets of strings vs. traditional tension. My jumping off point for this was Circle K strings, and you can read about it here. One snag: they recommend against using their strings in basses with through-body stringing. Crap. My ESP B4-E was a through body design. I plowed through pages of string sets looking for something near the gauges I was looking for:
C# = .110
G# = .080
C# = .060
F# = .045
(I arrived at these values using CircleK's tension chart, and doing some best guess rounding for what was available. I still wanted to be able to tune up to standard tuning, which is why my heaviest string cheats to the light side.)
If only someone offered bass strings for sale piecemeal, I could do what I wanted! Well guess what? Somebody does. I ended up ordering strings from Bass Strings Online. (Go figure). They have strings from lots of manufacturers that you can put together into your own sets. They were pretty inexpensive and shipped fast. I settled on Dunlop stainless steels for my first set for a little extra clank. I also picked up a heavier set for my ESP B-304 that I use in D-standard and Drop-C:
C = .120
G = .090
C = .065
F = .050
Which reminds me, I really need to do a post about the B-304. It has become my favorite bass of the ones I own, and I intend to make some alterations to my B-4E to try and get it to sound more like the 304. Until then, go buy some strings!