Actually, my findings Tom are that for your second point the opposite is true. Adding the multiple sources as you describe actually negatively affect the peaks/nulls. Multiple sources placed correctly using the same drivers will do as you mention.

Like you I also run my m80's below 80hz and use 60hz crossover.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85