The stock subs are crappy little things. You want MM full length subs welded in with the car on a lift supported by the tires, not the frame. Makes a big improvement to the stiffness of a vert.
I just got the MM full lengths installed on my Cobra a couple of weeks ago. $125 for the connectors, $125 for the install, totally worth it. The car has Steeda springs on it with 03 Cobra Bilsteins, and you could feel them working against the center of the car. Now with the subs on it, it is a totally different car. I drove my 91 for a couple of years without subs on it, and after I got them installed I vowed that sub frames would be one of the first things I do to a car from now on.