my dad's system is messed up

my dad's system is messed up it was workin fine yesterday its a 1999 saturn sc. i just took tha 2 10's out an put a 12 in no big deal but when i turned it on an had tha vol. low tha speakers sounded stadicy but when i turned it up to 20 it was fine anthing less than 20 its stadicy. i've had tha speakers hooked up to tha 4ch. fosgate 851 amp for a few years now an this is tha 1st time a problem came up. i'm using 2 ch.s for tha 4 6.5s. also there is no problems wit tha sub. any help would be apprecitated cuz i've been trouble shootin for a lil while now.tk

a little more info might help... HU?  OEM or Aftermarket?   Static on all functions? AM/FM & CD?  etc   etcKevin Gerry

Certified Electronics Technician

MECP First Class Installer

Owner/Installer

Classic Car Audio

since 1979

its an aftermarket system sony head unit, sony 6.5s. also i switched head units with a jvc an it was still there in tha speakers (the static). yea theres static with tha radio, cd an ipod functions. but today i switched tha ch. tha sub wires were in an it stopped tha static an now its fine but i put tha wire back in tha 1st ch. an it was still fine (no static). i have no clue why this freak problem happen.tk