It's very much horses for courses. With REW you can go a long way but it takes time and effort to get the FR curve that you are after, and if you want to indulge in phase/timing correction as well that is all possible to do using the FIR capabilities in the 2x4HD if you chooses the right tools to help you generate the necessary filters. The beauty of the Dirac approach is that you reach the goal rather more quickly - draw the target curve, do the measurements in the listening area, generate the filters, job done. So with Dirac you can very quickly reach your desired FR curve, and have it fix the phase/timing stuff, without busting too many brain cells. I'm absolutely sure that with sufficient time and effort you could reach the same end point using REW and other freebie tools, but you might be still at it in a few month's time. These days I use a bit of a hybrid approach - I measure with REW and tweak the PEQ filters and crossover settings in the DDRC-24 until I have a pretty good "fit" between the drivers and have ironed out the grossest of the peaks and troughs in the FR curve. I then run Dirac to apply the final polish. Using that approach, the difference between Dirac on and Dirac off is (not surprisingly) more subtle, but still gives a very significant improvement to my ears.