I totally agree with the solution given by revolt. First make sure, as the previous guy pointed out that its not just a bad room. Check it on headphones if you have a decent pair or cut a copy and just check it in another room on different gear. Then, assuming you mean a literal B note and not a downtuned B string or anything, use a parametric EQ on a high Q setting of about 8 - 10 to cut the frequency its occuring on. If its B then you should find the resonant frequency it around the areas of 120Hz / 60Hz or thereabout on standard guitar and 30Hz for bass. If not then use multiples of these numbers to help find it. Use as deep a notch as you need to control it. If its still there even after deep notching then id suggest something to do with your gear. In fairness I would always use a High Pass filter to get rid of absolutely everything up to 60 on bass and up to probably about 60 - 111 on guitar so filter first next time!