Well it's definitely not a mic preamp thing, that I can assure you - sitck the 57 pretty much with the center of the tip of the mic on the edge of the dustcap in the center of one of the speakers in your Recto 4x12, and then work on dialing in the Recto preamp, more than likely it's the settings there that are at fault! (I've been working A LOT this past week on tweaking my Recto, it's got some powerful tone-shaping options, for better or worse!)
What are your settings btw?