Well I am almost afraid to venture on this ground, but what the heck why not. As someone who has owned a SWR megoliath 810 and now has a 1515/66/1, i have done a side by side before, And i have to Say they are Both amazing Cabs!! I loved my 810 and if i was to recommend one to someone i would recommend Megoliath 810, It is one hell of a cab and can Leave you vision blurred and you pants soiled

When it come to overall volume, wouldn't say my "66" burrys the 810 in Max SPLs, But it does get stupid Loud!! At the same time my 810 could not even come close to handling a low B with a good amount of Bass and sub bass eqing at loud volumes without farting all over the place. With the 66 I can lay into a Low B at loud volumes with a massive amount of Low End EQing and not get it to burp!!
......... NOW FOR THE CATCHER!!......
In a real life setting on a large stage, I am not eqing the snot out of my lowend due to the fact that i would get nothing but a blurry mess of mud. Yes The 66 is more clean and clear than any 810 cab i have played through when it comes to eqing it low, but things still will get muddy. As someone who eqs my rig Flat across the board (i know so exciting) there is really not much of a difference between the 2, say for the Off Axis dispersion of sound from the 1515/66/1 and compared to the 810. The 66 i find to be more balanced no matter where i am on stage, and there is not much change in sound no matter where i am at which is great. Whereas the 810 had more hot and Dead spots when moving about on stage. But really in the end i stand in front of my cab most all the time anyways so that really does not affect me as much as it did my band mates.
All And All Both Cabs are Killers in their own right. For me i prefer my fEARful, and most everyone that has played through it has LOVED it...esp sound guys, but they are not for everyone the same way an 810 or a 310 or an X10 or X whatever cab is not for everyone ether! For me if there was No Such thing as a fEARful or any other such said type of Hifi 2 and 3 way cabs, I would have another 810 In a heartBeat

!! hope i did not muddy the waters even more, just want to give a real world perspective on it! Cheers All ~Elliot