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
manbehindthcurtain
Another possibility is going with adcom saperates.

http://cgi.ebay.com/ADCOM-GTP-500-II-Stereo-Pre-Amplifier-Tuner-Preamplifie_W0QQitemZ200375882470QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item2ea75552e6&_trksid=p3286.c0.m14

http://shop.ebay.com/i.html?_nkw=adcom+amplifier+555&_sacat=0&_trksid=p3286.m270.l1313&_dmpt=Vintage_Electronics_R2&_odkw=adcom+amplifier&_osacat=0

These will give you plenty of power - 200 watts per channel.
Thanks for the amp recommendations. A question about installation and setup-

Right now, I am using my receiver inputs for my digital cable and dvd player for surround sound. If I bought, say the Acurus DIA 100 amp, or any other stereo amp, would I be able to run the amp to the CD player, and still maintain my surround sound and stereo output for my DVD and Cable?
given that you have a two channel set-up, i'm not sure why you need surround sound--it seems to me that you should be listening to each source in (2ch) stereo. that being said, i concur with the foregoing that you'd be better served by ditching the avr and going to either an integrated or remote-controllabe seperates such as the above-recommended adcoms. you would then connect all your sources--cable, dvd, cd--directly to your integrated or preamp.
when You ask "when"...it is clear what you currently own,is
failing to satisfy, otherwise, the thought would never enter
your mind."The bloom is off the rose"?IMHO that is.

Love Your Music!
Why do you think ditching the capability for surround sound is a good idea? Because I am treating the bookshelfs and sub as mains together?

Ultimately I would like to move towards a home theater setup that will also do stereo. I understand though that the bass on the Mites is limited. I'm starting to feel like getting the Mites was a mistake, because theyre too nice to relegate to surrounds, but not big enough to power stereo listening.

Would something like the Marantz MM8003 provide a bridge to future growth in a home theater setup, but still provide power for the mock "surround" setup I have now?