Mac just to answer the other part of your question,There is an output for speakers on my computer .There is no left or right outputs on my computer just one out put .I then have a cable which fits the computer out put which is much smaller then 1/4 inch. The wire splits up into left and right with 1/4 inch jacks on the other end . I then have that connected to the input left and right on my external mixer. The power amp is connected to my mixer and then out to the Pa Speakers. Here is the link to this mixer im using ------->http://www.mackie.com/products/profx8/ I am connected to the 7/8 inputs one 1/4 jack on the top and one 1/4 on the bottom input.This is the way Mackie tec support told me to hook it up.

Regards,
Greg