$'objectname'.modifiers[#modifiername].enabled = true/false

It is sometimes useful to have different modifier enabled or disabled for different passes. An alternative to using a different objects via visibility sets is to use the Before/After scripts to enable or disable the modifiers.
 
Modifiers can be accessed via MXS using either index (from the top), or by name - name is obviously more reliable since adding modifiers will mess up the index.
 
One way of getting an objects modifier by name is:
 
$'objectname'.modifiers[#modifiername]
 
(where 'objectname' is the name of the object and #modifiername is the modifier name - it pays to rename the modifier so you don't have multiples with the same name) and then  
 
.enabled = true/false
 
You can of course also modify other properties.
 
*Note that the ' used either side of the object name is useful - it enables referencing objects with spaces in their names (not that I personally like spaces in names...)

RPManager : Control modifiers on a per-pass basis

Maxscript