For those of you that may have or are thinking about adding a 3rd sub to your system, I want to share my process that worked for me. For my 3rd sub I went DIY this time around, just something I wanted to try and it's not for everyone. The new addition is an UM18-22 driver in a 2' x2 'x2' sealed box made out of 3/4 inch Birch Plywood powered by an Inuke 6000DSP amp. Ok, the setup process spanned over 3 days with 2 nights spent messing with the subs until 4 a.m. I first tried all 3 subs up front which was a disaster. I could never get all 3 subs to play nice with each other, too much canceling. I ended up throwing the DIY sub in the back left corner and the S3600I and S3000I up front, left and right side and spread out. I then did the steps below:
1) I made sure all 3 subs hit 75db EACH.
2) I ran a sweep of the S3000I by itself and then ran a sweep of the S3600I by itself.
3) I adjusted the delay ( several times) of the closest sub to the MLP ( my seat) until I was getting 4-6db gain through the measured frequency. Note: It's ok if at a certain frequency you get no gain at all and usually it's only covering a minute range where you would never notice it.
4) I turned the front subs off and EQ-ed the UM18-22 sub as flat as possible ( except for a 35-45 peak) I was able to get it flat down to 5hz.
5) I then turned on the S3600I and took measurements of it along with the UM18-22 DIY sub and I got a nice flat graph down to 5hz.
6) I turned the S3600I off and turned the S3000I on and ran a sweep ( DIY and S3000I) and another great graph down to 5hz.
7) I then turned the S3600I on and ran a sweep with all 3 subs and that's when the fun starts. My graph was no longer flat down to 5hz, I was getting a nasty rolloff/dip around -16-17hz and I was getting canceling at certain frequencies and not gaining much with the 3rd sub so it was time to put some work in.
8) I started with the S3000I as it didn't have any delay set so I slowly increased the delay and ran a sweep with all 3 subs. I did this several times until I could not get any more positive results out of it. The graph look so much better after this process.
9) I then decided to see if I can further improved things with adding more delay to the S3600I, remember I had already added delay when I was dialing it in with the S3000I so I was afraid I might actually cause more harm than good by adjusting it further. I made a note of where the delay was set and then I slowly added a little more delay and ran a sweep, with each adjustment the nasty dip at 17-16hz was improving and I was getting positive gains in other frequency spots. I kept increasing it until I saw it starting to have a negative impact.
10) While I was able to improve the dip at 17-16hz, I know if I had a 4th sub in the back left corner I would be back to being flat at 5hz so the journey continues
I hope this helps anyone currently trying to dial their subs in as placement and the delay setting is key along with a Mic and REW, without it it's impossible to get your moneys worth out of this hobby.
NOTE: I still have to run Audyssey and see what my hard work nets me