Look at Boavista Palace.
We stayed there last week for a week. We have stayed there at least twice over the past 2 years when in Porto.
Located in the Boavista area, only two blocks from Carolina Michaelis Metro and an 8 minute walk from Casa da Musica. It is very easy to access most areas of Greater Porto from there.
See some reviews here: http://goo.gl/qKga7
