My prog band has 2-3 songs in Bb, most material we play is in drop-Db. For the Bb stuff, I have a 5er that is tuned 1/2 step lower than normal. I can play most of the drop-Db as if it were in D and not have to change basses to play the Bb tuning. Our guitarists don't have that muddy, sludgy, St. Anger tone either. I hate that tone, and I was worried they would sound like that when they gave me the material they were writing, but they sound really nice, just deeper. FWIW, my Spector sounds GREAT in this context. Their aggressive nature really shines in this type of material and the low Bb slams in the mix.
dave64o, people have been using alternate tunings forever. It is not just a metal phenomenon. Many solo acoustic guitar players tune their acoustics all the way down to B or A for a fuller sound. Its not better/worse, heavier/lighter to me, its just a different sound. Its one more thing that can help you stand out in a crowd. However, once you get lower than A, I really think you are sacrificing good tone tuning low just for the sake tuning low. It gets really muddy and indistinct down that low.