a 50W sub might be rather underpowered considering your M60's and VP150's are quite powerful. A sub will definitely full up the sound in the lower end, especially on explosions, or any music with bass. Remember, a sub isn't just for rap and hip hop so it can thump away and make your house vibrate while you can't hear any midrange or treble. A good sub will really complement your system. I would suggest the EP350 by Axiom, but if you are on a tight budget, the EP175 will definitely suffice.