Also 30Hz is a somewhat arbitrary figure to set a HPF - the cut of every instrument should be decided on its own merits. For instance when HPF a bass I've ended up with cutoff points of anywhere between 30Hz and 100Hz. It really depends on how much low energy the song requires. If I've recorded a guitar part consisting entirely of chords played high up the neck, I will set quite a high cutoff, maybe as high as 250-300Hz if things are already a little crowded in that frequency range. The fundamental of the low E note on a guitar is 80Hz, but even if the part contains that low E you don't have to set the cutoff that low if the mix is crowded, because the ear will still make out the note from the harmonics.  Remember it doesn't matter if the instrument sounds on the thin side when you solo it, it's how it works in the mix that matters. Vocals too - you can get away with quite a high cut (especially for female vocals). I usually sweep the cutoff upwards until the point at which it's just starting to sound a little too thin in the mix, then back it off a little. I hate the sound of too much energy in the low mids, it makes my ears water. I just find that everything sounds a lot clearer in a busy mix if unnecessary low mids are cut from instruments that don't strictly need it. 
 
I've always been a little shaky about what kind of slope to use where, although I do find myself using a shallower slope with higher cutoff points and steeper slopes at low cutoff points.