I can't tell you where to stand - I'll just advise you to try MOVING AROUND - ie. not standing in any specific spot - to shift your perspective a bit. If you are sitting in a place where the bass is boomy, you need to get away from that place every once in a while. Find a spot where the bass ISN'T as boomy. If you want to experiment, run some sine sweeps through the low frequencies (40-400 Hz) and move around the room. Find some spots that sound balanced - or at least can compliment each other.
Re. vocals being added: I wouldn't do much mixing or even reamping without the vocals - it makes absolutely no sense. You will likely want the vocals to take center space, but I think this is where you're currently trying to place your guitars.
This. With the vocals in the mix, the whole thing will change drastically. Mix only when you have all the components of the song in place.