I would never recommend having a front firing subwoofer with the driver facing the wall (if that's what you were saying you tried). That would just excite a lot of structural vibrations. You probably already have a lot of vibrations that need tamed with those beasts.
In the front corners (drivers facing seats) I'm sure you got great output but may get a tad muddy or bloated. With them on the inside of your mains, I'd bet you had to turn them up 1.5-3dB each to compinsate but get cleaner bass that way. Plus the advantage of having the mains further apart is always nice!!
I used to have my mains about 8.5' apart when I had some of my other smaller subs. Once I got my VTF-15H & found the best spot for it in my room, it made me have to move my front right over about 1'. Since I'm anal about the mains being the exact same distance from center I had to move my front left in a 1'. So now I have them in a lot closer than before. Suprisingly, I haven't noticed the sound being any different or narrower which I'm glad of. It LOOKED better before with the mains further apart & dual smaller subs inside the mains, but it SOUNDS MUCH better with the larger sub even though the mains are closer in. That's the important part...how it sounds!!!
Nezff,
Have you noticed any change in the eD subs since you first fired them up?
If you have, could you describe the change?
How would you compare the 18"s to your previous 15"s?