How do I know if I need an amp?

My system is as follows:

Marantz SR8200 AV Receiver
Totem Mites bookshelf speakers
Totem storm subwoofer
Onkyo DX-C330 Analog ouput disc changer

I've got the speakers wired directly into the sub and treat the bookshelfs and sub as "large main, left and right" in the receiver's output setup.

My living room is quite small, I listen to the system from about 8 feet away on the couch.

What kind of changes to listening quality could I expect from buying an amplifier for the system at this stage? I don't plan on buying new speakers anytime soon, so I am more interested in the incremental clarity that I could achieve over a need for any additional power.

I'm not an avid movie watcher, so I wonder if a 2 channel amp would make more sense, though the marantz PM5003 I saw ($600 is about my price range) only puts out 40watts, whereas my speakers are rated up to 80 watts, and the receiver I have now puts out 120watts.

Is there a big quality difference between the SR8200's 120w per channel and a stand alone amplifier's 40watts? Would a 120w amp, used in a stereo set up, make a big difference?

Thanks