I know SPL isn't everything but 8db in-between the two I would consider significant. The only reason I wouldn't pick up JBLs is they are $500 more expensive per cab.
I bet you're right about the higher output of the JBL's. You can't always trust spec sheets though, and the Alto's are typically rated pretty conservatively. I just haven't listened to either speaker so I can't comment on that. I'm sure someone else can.
I do like the idea of putting them deeper into the space on delay. I assume outdoor events don't suffer from comb filtering because the sound doesn't have anything to bounce back on, and I already have 2 outdoor gigs to prepare for so using 4 might be favorable.
While reflections
may cause comb filtering, it's the speaker spacing that I'm referring to. When you have 2 sound sources spaced further apart than ~1/4 wavelength of the highest frequency of interest, you will have comb filtering. Typically this is only an issue with the mid and higher frequencies due to their shorter wavelengths. When splaying speakers so that there is little overlap between their HF horn patterns, the comb filtering is minimized because most of the HF goes where you point the speaker. When pointing them the same direction, there is overlap so the sound waves will cancel or sum depending on where the listener is.
The lower frequencies tend to combine well because their wavelengths are much longer than the physical size of the speaker boxes being used.
Some speakers are designed to array well together. These will typically have narrow horizontal coverage patterns so that they can be tight-packed without much HF overlap.
Try sitting your 2 Alto's right next to each other pointing the same direction. Play some pink noise through them both and walk from side to side. You'll hear what I'm talking about. Then try splaying them wider and wider until you can't hear such a pronounced "ripple" when you're between the 2 but not too far such that you lose all of the HF in between.
All that said, comb filtering is not the end of the world and many in the audience won't notice or care but that definitely doesn't mean it shouldn't be considered. It's one of the many, many aspects that contribute (or deduct) from the overall quality of a sound system setup.
