Kenwood kdc x689 Something doesnt sound right?

Posted an earlier question about my 2 zone radio and the new Alpine sps 17c2 speakers that dont seem to sound right. When the 2 zone feature is on, only the front works and it sounds pretty dam good. But when off all speakers are enabled and doesnt seem to sound as loud or have is much b. The previous stock speakers in back were 4 ohms just like the new ones. Please help a guy out here. Already checked and rechecked the polarity. Chime in please