Strings tuning advice

[Glux]

Hi guys, I have a really annoying problem with my bass strings and was hoping someone could give me some advice.

I play in a modern hard rock band, it sounds really heavy and agressive and I tune my 5 string, like the guitar players, in Db, like this:

5-Bb
4-Db
3-Ab
2-Db
1-Gb

It really is necessary for some of the riffs to sound good, plus I started playing bass when I joined the band so I`ve pretty much always played like that, but the thing is, as you can see, the strings are tuned lower then they`re supposed to. It doesn`t make that much difference in all but the fourth string ( Db ), which is tuned 1 and a half step lower than the usual and stays too loose on the bass and sounds awful. I`ve tried to make up for the tension buying thicker strings of all kinds, but was never fully satisfied, until finally a friend of mine who owns a music store suggested I should put another B string instead of the E, so my fourth and fifth strings were about the same thickness. I`ve been told it shouldn`t be a problem and it shouldn`t damage the instrument as long as it`s set up properly, so I gave it a try. So far I think it sounds great, but the setup is being a bit of a problem, so I'd like to see if anyone has any advice on it, maybe some other sollution, or know of other players who've done the same thing. If it matters for anything, my bass is a 5 string active fender american deluxe jazz bass. Thanks.