may be a tad obvious but to really hit that sweet sub bass sound key is a major factor, I find (and sub focus has mentioned this) that writing tunes in the key of F and G really hit the low frequencies nicely anything higher in the bass register and your really forcing a higher frequency note to be lower and this is when things get messy and don't sit well - I am working on a tune now that started off in C and I just transposed it all down 4 semitones and the difference in bass intensity is amazing - and thats just 4 notes - anyways just a thought if your writing in a higher key.