That's a big group of people and it sounds like fun!
As far as increasing the volume vs. the crowd noise, that's a dog chasing its tail kind of problem. You might try going in the opposite direction, lower the volume, and see if people try harder to listen. Your speakers seem to be fairly efficient with a 90dB rating so your fairly efficient with regards to amplifier power.
You're on the right track wanting to get more granular control over the individual channels, but you already have that
. It's on page 28 of the Denon AVR-1706 manual. What you want to do is refer to the manual and increase the center channel volume by 3-6dB vs. the R/L. That will help the center dialogue tremendously. It's not an uncommon problem with an outdoor set up. I run my R/L and surrounds at 0dB and the center at +4-5dB. Another way to do it is to reduce/decrease the R/L and surrounds level 3-6dB and leave the center at 0dB. Either way will work and the master/main volume control will keep the ratio increasing/decreasing all channels.
You'll get a substantial bump in sound quality, using blu-ray and lossless codecs, by changing out the AVR. A new AVR that supports lossless codecs, Dolby TrueHD, DTS-HD, and DTS-HD Master audio will make a substantial difference to the over all sound quality. It's also an easier set up using two HDMI cables, blu-ray to AVR to Projector. You might want to take a look at Accessories4less and and pick up a refurbished unit, it's what I did for outdoors. Anything around 75-100 watts will do the job. You can double the power output to 200 watts, but you will only get a 3dB gain for the trouble.
You can go, or if you go with 2 PA speakers, the audio will have to be down mixed to stereo and 5.1 does add a lot to the fun to the movie. I have a 5.1 out door theater set up using speakers that are about the same size as yours. I live on a 1 acre lot and I have no trouble filling it with sound with an 80 watt AVR.
As for screens, I would recommend you replace yours with a Carl's 214" Flexiwhite. You'll love it and it will end your water spot problems.
If you want to switch to rear projection, Carl's has these two.
EDIT: Are you located in the UK? I ask because of the speakers as they're not so common in the USA.