PA System Setup Help

Hi, so I just bought a PA system over the weekend and brought it up to school (IUP) with me so I can play music at house parties, and DJ in my free time. I bought the system to start off playing music off my computer using Virtual DJ and eventually transition to playing on vinyl. So, I bought really nice speakers and an amp to put out the sound and I will get decks and a better mixer later. When I set up my system though the music sounds hollow. Some songs I will hear the background beat and no lyrics. Sometimes you will hear some lyrics but only the really loud parts and the background vocals will never come through. Put it this way, my $60 Altec Lansing computer speakers sound better than the $1200 JBL PA System that I just bought!! What did I do wrong??

Here is my setup and what I do, please help me to figure out if my problem is my lack of skill, or if it is something technical.

Equipment:
2 x JBL JRX100 - 115 Speakers :: 1000W/250W Continuous
1 x QSC GX5 Amp :: 500W / Channel
1 x Alto ZMX52 Mixer

Setup:
2 x Speakers -> Amp -> Mixer -> Computer

Speakers to Amp: Regular PA system cables

Amp to Mixer: RCA (Amp) to 1/4" Jacks (Mixer)

Mixer to Computer: 1/4" Jacks (Mixer) to 3.5mm (Computer Headphone Jack)

Here is my process: I turn on my computer and connect everything. Set all EQ to 0, Gain to 0, All levels to 0. Turn on mixer. Set all Amp levels to the lowest possible. Turn on Amp. Play music and slowly turn up levels and gain. But once they get up, the music sounds terrible. Am I setting the Gain wrong??

Please help me, I just want to know if it is me being bad at using a mixer, or if theres something wrong with the equipment.

Pictures of setup:

Front View:
http://imageshack.us/photo/my-images/703/img00132b.jpg/

JBL JRX100
http://imageshack.us/photo/my-images/39/img00142.jpg/

Back of Amp (GX5): Left: RCA Cable to Mixer. Right: Speaker Cable
http://imageshack.us/photo/my-images/542/img00152.jpg/

Mixer (Alto ZMX52): Left: 1/4" Jack to Computer. Right: 1/4" Jack to Amp.
http://imageshack.us/photo/my-images/808/img00162i.jpg/

Shitty Closeup of Mixer:
http://imageshack.us/photo/my-images/192/img00172v.jpg/

Better, but still blurry closeup of Mixer:
http://imageshack.us/photo/my-images/827/img00182y.jpg/

Cable into Computer ^.^
http://imageshack.us/photo/my-images/837/img00192q.jpg/

Some Speakers:
http://imageshack.us/photo/my-images/259/img00202y.jpg/