I've done some long, boring maths and fair bit of thinking.
I am struggling to understand how 'mismatched speakers' in a bass stack can make a significant impact on live sound unless one of them is wired out of phase.
Two reasons:
1 - using two identical speakers could actually be more likely to cause worse dead spots as you need two identical signals to completely cancel each other out.
2 - at the frequencies of significance to most bass the path differences between the elements of a stack is too small to cause destructive interference even at fairly close audience distances, especially if they are mounted one above the other.
The only mechanisms I can see affecting sound out in the audience are:
1 - Beaming.
2 - refections around the venue
3 - interference with the PA sound if the backline is of comparable volume.
But I can't think of any of these that would be made significantly worse by using 'mismatched' speakers any more than using matched ones, other than the extremes of beaming an extreme such as Trace Elliot 'bright box' sending high frequencies over the top of the front of the audience.
I'm happy to admit I'm flailing around in the dark with these ideas so if anyone can explain how mismatched speakers cause problems, please do!