OK, does the hiss happen when you just turn the amps on with nothing being input, or only when you plug your gear in?
The older Mackie amps were great, some of the newer stuff has been a bit noisey, so gain-staging is very important. If the hiss is coming from your gear, quality cables can make a decent improvement (it sounds like you have standard RCA cables coming from your Pioneer; see if you can get the heavy-duty cable with gold tips), and you'll need proper adaptors to plug directly into your Mackies. As I mentioned, gain-staging is important, so you'll have to mess with the relative volumes of your gears' output in relation to the volume on your Mackie speakers until it's as close to "right" as possible. It might be your cables, it might be your mixer output, or the Mackies, the volume settings, or a combination of those factors. Try trouble-shooting one thing at a time (starting with the cables), and get the optimum settings on everything (and make notes or actually put a tiny mark on the gear to remember settings), _before_ you start fiddling with EQ. A lot of times, this can be solved by gain-staging, before you have to destroy your music material by cuttting the highs out of everything just to reduce the hiss. I have an older Mackie 808, and I had some problems with it and a little mixer I use for DJing gigs; the long and the short of it is, I finally figured-out the volume setting issue and everything's pretty optimized now. So spend the time to figure it all out before you bring the stuff to your first gig. The new room and the crowd or lack thereof will be a whole new sound issue to deal with, so if you've at least got things dialed-in from a generic standpoint, it will give you a good starting point.
Hope it all works out!
GJ
Oh, PS Again-- Your computer might also play a roll, depending on how you route things. If you hear any funky digital clicking/glitching sounds, or the hiss is coming from a computer output (not sure how you have things set-up), you may need to isolate the computer power supply or get a better cable for that too.