Rais.007

so ive been wanting more bass in the system, ive read a few posts about this and im trying to see if anyone has upgraded their B&O sub on a A7/S7/RS7 with the original sub box and exactly what they installed, i just want a plug and play sub, i don't wanna deal with new amp, loc, etc... just simply plug and play. my stock sub is blown, so i went to my local audio store and got a kicker 8" 250w 4 ohm sub but the system shuts down on little over than 18 volume, and only the tweeters play sound, i believe thats overpowering the stock amp. id like to know what should i but so my amp will work flawless with the new sub.