<3 for z-640s. First off, the sub is supposed to have a dent in the front mesh, if it is exctally in the center and perfectly round, then thats how they all come, otherwise, that sucks but i is probly fine.
The rear speakers may not be pluged into the right spot. I had a similar problem. Try plugging the black minijack into the orange plug and the orange into the black. Also, try messing with driver settings. When i turn on dolby surround encoding on my nf7-s i have that problem, that, i think, is the actual cause, the swaped minijacks was another problem. These things were a pain to setup but they sound great and are sweet for the price.
oh yeah the bass is decent but at higher volumes it cant do much, part of this is the location (the middle of the finished basement isnt the best place to ressonence) but its not bad for the price at all.