I want to get a Bennelli M1 Super 90 with the Pistol Grip Buttstock.  Is it legal to add the magazine extension tube if I have it permanently affixed?  Thanks

No!

(D) a semiautomatic shotgun that has at least 2 of-
* "(i) a folding or telescoping stock;
* "(ii) a pistol grip that protrudes conspicuously beneath the action of the weapon;
* "(iii) a fixed magazine capacity in excess of 5 rounds;
* "(iv) an ability to accept a detachable magazine.".

I just wanted to add that we are currently searching for the same rifle.
I was just pricing a Bennelli M1 and can't figure out if I want the pistol grip or the 8 round tube.
I kinda think that the pistol grip is going to win out, its a little more comfortable for me.

I have a Benelli M1 with a standard stock. I found the pistol grip stock jumped more and it took me longer to get back on target.

I have a 24in "bird" barrel and a 18in slug barrel. I use it for bird hunting and defense. I have tried shooting clays (and birds) with a pistol grip and found I was faster with the standard stock. I sold the pistol grip. I do have a magazine extension.....in defense mode I feel that is a MUST have.