Sunday dining in Madrid
I think most of us tend to eat a big late lunch on Sunday and just have a snack for dinner. In my neighborhood (La Latina) I think most of the tapas bars close in the late afternoon after the post-Rastro crush.
Since you are looking for a Sunday place and a seafood place, you might want to combine the two and check out Marisquería Rafa (near the Retiro):
If you do some googling, I'm sure you could find some reviews in English. You'll find simple, traditional sorts of preparations there with very good quality raw materials. It's still not cheap, but perhaps less expensive than the other marisquerías you are looking at.