I was in Sofia for 1 week in March. I stayed at Hotel Arte which has very good prices and location. From there you are close to everything. I found a few nice clubs just walking around the area. Vitosha blvd is a great place to start. I stumbled onto a piano bar that had great entertainment and dancing. Also there are disco on Vitosha.

check out the nightclub 4km, do a google search 4km.com proper BIG club

have fun, and take care