Try all of the following in the following order:
1. For a perfect equilateral triangle, you should be 5.5*tan60=9.5 feet away from your monitor. Half a foot makes a big difference. If you can't move back, then place your speakers 2*9/tan60 apart or 10.4 feet.
2. Toe them in so that the tweeters are aiming about a foot behind your head. To ensure the perfect toe-in, some use a laser interferometer
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3. Calibrate with an SPL meter as suggested above.
4. Then, re-calibrate using your ears. If there is more open space around the front left, you will have to decrease the front right and increase the front left gains. I have the exact same issue (see picture below) and had to do the same thing. Keep tweaking with the channel levels until the image is in the centre. The pink noise calibration doesn't work well when you don't have symmetrical reflective surfaces.
5. Find the correct setting for your 500's phase switch. Here's how. Set your receiver's main volume control to your normal listening level (mine is -20dB). Turn off all your speakers and leave just the sub on. Set up the SPL meter right at your sweet spot on dBC, slow. Play a test tone that exactly matches your receiver's cut-off frequency. So if it's 80Hz, play an 80Hz tone. Flip the phase switch one way and then the other. Keep the setting that results in the highest SPL. In my room, there is as much as a 15dB difference between the two settings.
6. Make sure your front speakers and your sub are matched at the cross-over frequency. With your front speakers still off, record the SPL. Now turn the sub off and turn your mains on. Record the SPL. Now turn the mains off and turn the sub on. Using your receiver's sub level control, adjust the volume of the sub channel to match the mains SPL.
5 & 6 above sound complicated but they really aren't. I'll be posting separately to explain more about this.
The fact that your M80 tweeters and centre tweeters are not aligned horizontally is not good for panning especially since you are sitting somewhat close. Sorry, I don't know what to suggest about this. Maybe someone that has more experience with this can help.
