OK Thanks, you've answered one of my questions. Since I normally have my tops over my subs, I haven't been worrying about the system optimizing but I guess there's no guarantee they are in sync as is. So how do I measure that and what type of delay do I buy to delay which ever send (tops or mids) I need to delay.
The classic audio answer applies: "It Depends...."
A "system controller" or "system DSP" is typically used for this. Examples include the dbx DriveRack series, BSS OmniDrives... I think Behringer still has one, also. There are a significant number of choices that didn't exist 10 years ago. They all pretty much do the same things (input EQ, compression and delay, output EQ, delay and limiting) although some are better or more user-friendly than others.
If you're using the same brand of tops and subs I'd expect the manufacturer to provide you with information about any needed delay of tops or subs relative to one another when using the manufacturer's recommended crossover parameters. Note that while we often hear folks talk about delaying their subs, more often the tops actually need a bit of delay. Why? Because the high-pass filter (at the lower end of the sub's range) that helps protect the speaker also introduces some time delay as it does it's job. The lower the frequency and steeper the filter (more db/octave), the greater the delay.
In the world of system optimization, you do the above alignment first. Make the tops and subs work together as best they can. THEN you can apply full system delay (at the system controller inputs) in the manner Jay talks about. Whether or not delaying the mains to the band's stage sound will help you depends on how close the PA's SPL is to the band's stage SPL. Jay mentions 6dB and I think that's the right place. Also important is how much of the venue is covered by BOTH sources within that 6dB window. For most weekend bar gigs I'd skip trying to delay the PA to the back line instruments, but you can experiment once you have a way to do the delay. A good estimate (for starting purposes) is 0.9ms per foot of distance between the PA speakers and the loudest thing on stage that is in the PA (and in a small room, EVERYTHING probably is picked up by the vocal mics, which presents other time issues that will complicate our discussion
) If you stand equally between the PA speakers and match the level of PA to band stage SPL, lengthen the delay time in small increments until you hear the sound image come together.... meaning you don't perceive the PA and stage as being separate sources.
That said, your delay alignment is only valid for the spot you stood in. The delay time needed will change based on where the listener is relative to the stage and the PA speaker covering their spot. You can only optimize delay settings for ONE SEAT IN THE HOUSE. Everywhere else is a compromise, and whether or not that compromise is worth the effort depends on the political value of whoever is in the sweet spot.
Have fun, good luck.