Discos Paradiso is probably the best music store in Barcelona. A great selection of vinyls and rarities, as well as being a hang-out for the local DJ community, means that Paradiso is ahead of the pack - at least when it comes to different electronic expressions.
If you're not shopping for music, Paradiso is always an excellent place to find flyers and that kind of information about goings on in Barcelona. Hang around for long enough, and you might just find yourself on the guestlist for that party.
Watch out for in-store events.