I agree with papermaker... I have a 4 channel amp in my 451, one channel to each door speaker and two channels bridged to a sub... It couldn't hurt to have more speakers but the extra power to my sub sounds nice!
Either I might just not add in the two speakers rendering the 2 channels useless or might end up doing the rear speakers in the back as pictured below. -- I'm wondering what would the sound quality be like to have the rear vs. no rear. Also, if there are rear speakers, will it drown the bass compared to 2 speakers and 1 sub...?