Hmmmm...never had a tubby sounding 421 -- usually it sounds brighter than I expect, given that it's right next to the cone and my normal listening position is several feet away. I usually center it next to the dust cap (rather than directly over the dust cap) to cut down the brightness.
Could the wrong side of the 421 be facing the amp?
See how the "top" of the mic is pointing at the center of the tom? treat that "top" portion of the mic like the capsule on a 57. I originally thought it'd be positioned like an e609, grill parallel to the cab, but it's not the same.