There was a thread about this the other week.
For starters there is bass only
Jive Music in Arlandria (what us locals call the midpoint between Arlington and Alexandria)
www.jivesound.com
They are open between 1pm and 5pm and have good gear, well worth a look. Owner is Jong - a nice bloke!
Also worth a look is
Chuck Levin's (aka Washington Music Center) up in Wheaton - not far from Rockville.
www.washingtonmusic.com
One of my favs is
Action Music in Arlington. Usually got a few interesting basses (usually on 8 or 9 though) and if you're into guitar too there is always a lot of interesting vintage stuff - great staff too! They are open until 6pm
www.actionguitar.com
There are three
Guitar Centers - Fairfax and 7 Corners in VA and there is also one in Rockville. The 7 Corners store usually has the most basses.
http://www.guitarcenter.com/locations/
Looks like you're going to be busy on Mapquest or do you have Nav?
Have fun and let us know what you think or what you buy!