That's almost exactly the same setup I have - which is an Alpine 9820-XM, basslink and sony xplods (don't laugh, I know they suck.)
I don't have an amp and I was almost convinced to get one when I got the alpine. But the sound improvement was so substantial over the stock system, I soon forgot about it.
Now that i'm used to it, I wish I had the amp for clarity particularly at low volumes and with the windows open. But you might not have my problem - which is to say $hitty front speakers. The basslink is pretty good, though not as powerful as i thought it would be.