I also use ND filters on one enlarger with an Ilford head. On my Durst condenser I use a variac to reduce the voltage. Taking it from 110 to 70 volts lets me take off two stops. Each 10 volt drop seems to give about a half stop. Of corse you need an enlarger that uses a regular line voltage bulb for this to work. Also the color might shift a bit (and the contrast along with it).