When you say the Nathans are CD lower does that mean that the directionality is more tightly controlled or that they plumb lower frequencies? If the main difference is that they go lower it seems that the Nathans with some OB subs crossed over high would be a pleasing combination.
I think I read somewhere that the Abbeys have a slight hole in what would be the normal sweet spot and they actually sound better slightly off to one side. 
Thanks,
Roy
Roy,
All the GedLee speakers have a dip when measured on axis (at 0 degrees) due to diffraction at the lip/mouth of the waveguide itself. This has been compensated electrically in the crossover. But you don't
ever listen to any GedLee speaker at 0 degrees (except center channels and he compensates for this as well in the crossover). But for 2 channel audio, his speakers are toed in HEAVILY to 45 degrees for precisely this reason.
Earl explains it well earlier in this thread.The off axis response is linear as can be, and it's optimized that way.
Directivity control *is* enhanced as you go up in his designs. Although the Abbeys will be able to play louder because of greater piston area, etc...
that's not the point. The point is *directivity control*...which the Abbey has down to a lower frequency. If you look at the AES studies you will see that controlling directivity over a 30-40 degree arc from 500 Hz to 20khz is a very desirable characteristic in order to limit reflections, etc...it is what makes this speaker a soundstaging champion and tonality is the most neutral I have heard, on par with Quads in the midrange. Now the Abbey and Nathan do not control directivity all the way down to 500hz. That would take a massive waveguide larger than the 15 inch one on the Summa. But I will tell you that you can understand a lot about the *degree* of directivity control in each of these designs by looking at Earl's graphs on his website. The Summas are clearly the best, next Abbey, followed by Nathan. Again, I will say that the Abbey is better than the Nathan as I have heard both designs. The difference isn't night and day. But if it were my wallet, I would spend it on the Abbey only because the price difference between them to me is small. Just my opinion.
The Summas are just amazing.
The new cabinets are rounded over well - both around the waveguide itself and on the front baffle. You wouldn't expect anything less from Earl Geddes. Minimizing diffraction is paramount to his and any speaker design. Those who don't do so are ignoring that aspect of speaker design. The ear has a *non-linear* sensitivity when detecting diffraction levels at higher spl's - minimizing distortion caused by diffraction is key.
Reread his white papers fellas, it's all in there

Anand.