How do I know if I need an amp?

I would get an amp - it would improve the sound noticeably - but I would go with a different one than the Marantz PM5003. By the way, you can get one for $449 on amazon.com

I would consider getting it used here at the audiogon.com
You will end up with more quality for the buck.

Take a look at these :

http://www.audiogon.com/cgi-bin/cls.pl?intatran&1256328090&/Onkyo-A-9555

http://www.audiogon.com/cgi-bin/cls.pl?intatran&1256065250&/Music-Hall-a25.2

http://cgi.ebay.com/Acurus-DIA-100-direct-input-integrated-amp-NO-RESERVE_W0QQitemZ180385071434QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item29ffc9c14a&_trksid=p3286.c0.m14

These have more power than the Marantz, and would sound better in my humble opinion. I would recommend the Acurus especially.

Your ears will thank you if you upgrade the amp...

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.

What cables are you using ?

If you are using thin ones that came with the receiver, you might want to upgrade the cables first - and then see if you like what you are hearing.

You can get Ixos gamma RCA cables and Ixos gamma speaker cables at accessories4less.com That's what I use, and I like them. They got good reviews on hifi magazines.

http://cgi.ebay.com/Sensational-IXOS-Gamma-1002-Encore-Audio-Cable-2-Meter_W0QQitemZ140337596201QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item20acc5c729&_trksid=p3286.c0.m14

Also, you can add this inexpensive power surge protector - improves the sound, and protects your electric gears :

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=370249463085&ssPageName=STRK:MEWAX:IT

Your present setup is not bad - should sound pretty good, with the right cables.

I would go with a different amp than the MM8003 - there are better sounding ones out there for the same money.

I just listen to 2 channel set up even for movies - I don't miss other speakers. Since you are trying to improve the sound of music, I would consider getting the adcom preamp and adcom power amp.

They would improve the sound in a noticeable way. Also, the culprit might be your source - amp can't improve the sound a lot if the source is not good.

I would upgrade the wires (if you haven't already), then the CD player (or DVD/CD player), and then add a 2 channel adcom amps to your Marantz SR8200 for the fronts.

You can connect the adcoms to your present Marantz receiver, to power the fronts. That would be better than getting MM8003 - I would consider other brands than Marantz.

http://www.accessories4less.com/make-a-store/item/IXOSXHS606S/IXOS/XHS606-15-AWG-GAMMA-GEOMETRY-SPEAKER-WIRE-SILVER-PER-FOOT/1.html

http://www.accessories4less.com/make-a-store/item/IXOSXHA5162M/IXOS/XHA516-PC-OFC-Gamma-Geometry-Audio-Cable-2m/1.html