[eyekode]

Another thing that can be contributing to muddy vocals is poor EQ. I.e. your sub is at a much higher level then the mains. As looneybomber said lowering the XO may help the symptom. But to fix the whole problem you need to eq the sub and get the levels right. RoomEQ Wizzard can help. But if you don't have the equipment you can do it roughly using an SQL meeter and some sine waves. At least this will give you a rough idea.