I have been using the DRPA for quite a few years.
First, you need to make sure that your basic config is setup well. The cross-over point(s), amp configuration, and time delay between your different drivers need to be configured. There are rules of thumb you can use for the time alignment (for instance, my folded horns get ~8mSec delay to account for the distance traveled through the horn respective to the tops).
Second, there is a sensitivity setting on the DR's to allow a less precise eq.
Third, if your sub can't handle the lower frequencies (~30Hz) you will need to allow less precision for it to finish.
Fourth, make sure that the LP and HP are used on your sub. Don't send signals to your sub that it can't reproduce. You just end up working the amp and speaker harder and wasting headroom for power that you can actually use.
Fifth, outdoors is a bass sponge.
Sixth, ensure that your sensitivity and input gains are set properly or you risk clipping or loss of utilized power.
I have seen the saw tooth effect mentioned above and have always manually corrected it. It hadn't occured to me that it was comb filtering at work. I will try doing a single stack at a time to see how good the results are.
My system is all passive with Cerwin Vega LR36's on the bottom and Klipsch KP301's on the top.
If I bypass the auto-eq, I find that my system does not sound as good as it does without it and it would require quite a bit of work at the instrument rack eq to get it there.
I only use 2 presets. One for indoors, and one for outdoors. I don't like using the auto-eq on a gig because of the disturbance to the venue while I do it. I do fine adjustments with the rack graphic eq.
Most sound pros I have spoken to agree with the assessment of those here. No "real" sound-man would use auto-eq. Perhaps my talent doesn't extend as far as the "real" sound engineers

I kinda like the crutch.
As an aside, if you are using a matched pair of powered speakers, I would also agree that it is likely that the factory DSP is likely better than what you will be able to achieve with the DR; however, once you start mixing and matching, I am not sure this is still the case.
Many of these new powered speakers are DSP'd to sound good at low to mid volumes in a stand alone audition in order to attract buyers based on their "out of the box" sound of a CD. They typically have the smiley face eq going on which usually needs some correction in order to sound better with a live PA setup over a sub.