John, I saw the Gran Gala Flamenco performance at the Palau de la Musica a few years ago, and it was excellent. The main concert hall is magnificent. I got my ticket online, WAY in advance, and chose the front row center seat of the mezzanine, just perfect. Of course, a stage performance in a large concert hall is very different than traditional and intimate Flamenco which you might better be able to find in neighborhoods or small towns in Andalusia, than in Barcelona. (The Tablao de Carmen might be more similar, maybe someone who went there can comment about that.) On a different day, I also took the excellent English language tour of the Palau building, during which you see much more of the building, learn about its history, etc.
Enjoy wonderful Barcelona, with so much to see and experience!