[quote name='grunge666' post='1336457' date='Aug 11 2011, 02:19 PM']We do rock covers but, try and nail them as close to the originals as we can[/quote]
That's probably the problem. Judging from your username, you're playing heavy rock and grunge stuff, right? If so, then loads of those riffs and basslines are written with things like pull-offs to open low C, or playing a riff quite high up the neck, interspersed with open strings that are far from standard tuning. Not to mention drop tunings.
You probably could just about play some (or many) of them note-for-note on a BEAD-tuned 4-string (or CFBbEb or whatever), but chances are it'll be much harder work than sticking to the original tunings. Taking a popular tune as an example, it's equivalent to tuning a standard bass down a tone (say) and then trying to play Muse's "Hysteria" [i]in the original key[/i]. You can't rely on any open strings so it'll be awkward fingerings all around.
My personal suggestion would be to try transposing the tunes from the D tuning down to the C tuning, and the standard ones to Eb. Then you've only got 2 tunings to deal with, and you've halved the number of bass changes. (You could, of course, tune some up instead, but your singer might have issues with that. )
Alternatively, don't try to nail them as close to the originals as you can. Make them your own.