Strap the output transformer for 600 ohms. Follow this with a 600-ohm pad and finally, a 620-ohm termination.
The pad is necessary--the output level is very hot and without attenuation, would overdrive the snot out of your M-Audio box. Take a look at this photo album of mods I performed on a 436A; it includes a schematic of a stepped output attenuator giving 0 to -27dB (plus "off") in 3dB steps.
http://www.groupdiy.twin-x.com/thumbnails.php?album=77