I have NHT Classic 3 center and front speakers, and 4 absolute zero surrounds. The only non NHT speaker is the yamaha yst sw315. Should I buy a second matching sub, or sell the yamaha an get a NHT Classic 10? I'm using a yamaha rx-v663 for my reciever do 7.2 is an option, but i'm not super impressed with the sub currently. Thanks for any advice!!