For a 'one stop' shop (though it's 90% guitars), ochonamizu is the place to go, it's an entire music shop street..
Though for bass specifically I prefer Revole Bass Station in Akihabara.
There's also a really nice little pocket of shops (musicland, 246, ikebe) in Shibuya, just across from the Shibuya train station (there is a bus depot there, and a pedestrian flyover.. It's over there. I know you're thinking 'why us he giving me these details? I won't remember'.. You will, you'll go to Shibuya station, see all the busses, look up across and go 'hey there they are!').
Oh, with ochonamizu.. Don't drag a loved one along and allow yourself a whole day.