Sure, that should be possible:
- create a parameter from your drop-down list
- create new filter on the element you want to filter on
- edit the filter
- use the boolean operator IF...ELSE
- use the parameter in IF part with a comparison operator (I would suggest something like 'missing')
- insert your filter criteria if the parameter is missing (that means, no value selected in drop down)
- insert your filter criteria if the parameter is not missing (something like xyz contains parameter, xyz = parameter, etc.)
I have used a text box for something like this. I think this should also work with a drop-down list.
Cheers,
Michael