(click on pics for full size originals)
This is the schematic for the MG 3.7 and MG 3.7i. The schematic is drawn to show the wiring with colors. The schematics of the two speakers are identical but the 24.0 uF caps (bank of 3) had a change of manufacturer (Solen to RTI, respectively). The other caps remain the same (also supplied by RTI). Also, the 3.7 uF RTI caps are film & foil. The inductors are steel laminate which have DCRs of 0.1 ohm or less.
The resulting Bode diagram.
This is the frequency sweep that I obtained. The gradual downward slope from 500 Hz to 20 kHz of about 5 dB was intentionally done by Magnepan (discussion with Wendell Diller). The waterfall plot is also included. Obviously these are room dependent so YMMV.