problem with silicone sealant is that it's soft. it seals, but doesn't make the seal hard, it always remains soft. you want a hard seal. when you rap your knuckles on the sub box you want a tight high pitched "clink", not a dull low-pitched "thunk". the more rigid the box and seams, the tighter the bass. use gorilla glue, it dries hard. also consider internally bracing the box with scrap pieces of wood. that also helps increase the rigidity of most boxes. in fact, the effect of structually reinforcing a weak sub box can be like night and day. remember that it's the resonance of the box that produces most of the sound. without a structurally tight box, even the best subs will sound loose.