Following is a graph showing the frequency response of a late 70's slant Marshall 4x12 cabinet (checkerboard grille cloth, plastic handles) loaded with the original Celestion G12M25 "cream-back" speakers, miked with a Shure SM57 on the lower left hand speaker in the center of the cone, at a 45 degree angle towards the side of the cone.  Measurements were taken with a Hewlett-Packard 3580A spectrum analyzer.