Finally the fun part. You now have signals reaching the 5 inputs of the receiver, and have independent level control over balancing them. You can now actually hear something.
First run through the surround receiver setup and balance the system correctly as necessary. Basic stuff, forgive me for even mentioning it, but most of them are not set up right, even when people think they are. Check the number of speakers, presence or absence of a subwoofer, double check and set the correct distance/delay settings for all speakers, and run the pink-noise function to get the output level from all speakers as close as possible to each other.
With all that out of the way-
Mute or unplug the center and back inputs and play your recording with just the Left and Right channels. Adjust the channel level balance of just the spaced Left/Right omni pair alone. EQ them if you like. If it’s a recording made outdoors at a fest or amphitheater it probably sounds pretty good, just like a spaced omni recording should. Without getting into stuff like dynamic compression or stereo width manipulation, that’s pretty much the full extent of the adjustment you have over a straight 2-ch stereo AUD recording.
Once happy with that, un-mute the center channel and bring it up in level until it blends seamlessly into the Left and Right channels. It should make the entire stage across the front more solid like at the live event, improve the clarity and mid/treble range over the spaced omnis alone, improve the imaging all across the front, and make the recording sound more present and less reverberant, less distant. If you want to EQ the center to blend even better, mute the Left/Right and get the center sounding good on its own first, keeping in mind that that the bass in the center channel doesn’t matter and might even be better EQ’d down. Once you get the center sounding good, take it’s level back down, unmute the L/R channels, then readjust the center level balance as appropriate to blend with the L/R channels. Play around with the center level. You'll find there is a range of acceptable levels, within which you gain some nice control over the reverberant balance, the overall tone, the width of the front stage, and the solidity and dominance of the center which isn't available to you in 2-channel stereo. You can also globally change the overall tone with the bass/treble controls (like you can with stereo) across all three channels together, but unlike stereo, you also have control over frequency equalization of elements within the front stage by tweaking the center differently from the Left and Right sides. It's all inter-related. That's why I suggest its best to start by getting each part sounding as good as possible on it's own before combining them. That makes it's easier to find the magical optimal blend.
If all that EQ talk is too much, forget all that and just get the levels right. That's the most important part.
Once the three front channels are well balanced things should sound really good. Clear, big, wide, stable, a lot like it does live. At that point you can bring up the back channel routed to the surround speakers. The appropriate level (and EQ) will vary a lot more than the center, depending on the material, the recording environment, the audience and other stuff. The optimal level will also vary significantly during a recording. You’ll most likely want more at the start and end and between songs when it’s mostly audience reaction all around, more for quieter or simpler songs, less when things get loud an complex, less if the audience in back is obnoxious, or if the environment is overly reverberant. You can ride the level of it for your own listening enjoyment, or just leave it at the lowest acceptable level. A bit to little is better than a bit too much, but you will probably always want some level from that back channel. Even at low levels where you otherwise don’t notice the contribution from the surrounds, a front/back sonic dimension and depth will manifest within in the room, the whole environment will sound more natural and less 2-dimensional, the bass will sound more live and substantial, the music less anchored unnaturally to the front of the room as if listening through the window of a press-box. You may notice that you can listen with enjoyment facing sideways, and not just straight ahead, and that the image doesn't collapse to the nearest speaker as easily if you get up an move around. The best level for the back channel is one where you don’t consciously take notice of the sound from the back and sides during the music until it is muted, or until the audience reaction between songs comes in and totally envelops the room.
