None of the above. The problem isn't just the air, it's the surfaces.

Musty smells are caused by moisture and mold and other nasty things that are good neither for you or the stuff in the area.

If you have a carpet in the musty area, tear it up and replace it. If you suspect the area will always be a little damp, replace the carpet with an impervious surface. Run a quality dehumidifier and be sure to empty it often (or better yet, have it drain). Use a couple coats of Kilz primer on painted walls to cover up the stink there. Toss any fabric furniture and tear up the carpet. Once we did that in our house, our basement became habitable.

Also, once you do all that, make sure the musty area, if you're going to use it often, is in the airflow with the rest of the house. If you have central air/heat, it should have registers and intake in that area as well.