I have a Rockford 700's amp pushing two 12 inch mtx thunder 6000's set at svc 4 ohm so i was pushing 175 watts each sub. It was knocking hard for about 2 years. Recently the left sub blew. Did it get too old or what because those subs i believe are 4 years old because its been thru many people i mean i have my rca cables close to eachother in other words i have the rca cables of the subwoofer close to my interior amp sound rca cables so i have 2 amps and ever since ive installed the interior amp, the interior speakers have been whining i also didnt scrape the paint off for the ground wire.Thats my first question. Second question is im bought two 12 inch mtx thunder 8000's DUAL 4 ohms. I need some help in how to bridge it to (4 ohm load bridged (mono) ). Third quesion is why the cross over frequency knob doesnt make a difference in the sound. When i first bought it it made the bass stronger everytime i touched that knob for some reason, unless i was just hearing things. And i took it back to rockford to get it fixed and everytime i adjust that knob i dont hear any difference. Is it supposed to make a difference? thats about it theres 3 questions in there i need some help badly thanks.