I agree. You want to "zero in" on the improvement. Start with large extremes, I.e. +10, then +9, +8. there will be a value when the response is clearly worse. When you get there, start backing off in the direction of improved sound. And don't forget negative values as well. Point is, somewhere between -10 and +10 there will be a "best" value. If it is the same as the original Audyssey-calculated distance, so be it. Nothing wrong with that. I ended up adjusting Sub1 by +6.4 ft, and Sub2 by -.1 ft.
I'd went the other way--starting from the current distance and going incrementally up/down. It's worth a try to start with the extremes and go from there.
As to what to use with mains+center, what about 'All Channel Stereo' as an alternative to DLII, at least with the OmniMic and Denon? Isn't the Denon essentially cloning the monophonic signal to all channels?