This is a favorite venue which we often visit and have enjoyed seating in numerous locations. Congrats on planning to attend this often missed experience which typically is filled with locals.
Keep in mind the music is only a part of the experience as the architecture truly allows the word "unique" to be applied. The acoustics within the facility are great and it is also an air conditioned experience! My guidance would be to sit in the middle of the balcony so you can absorb the full experience. This will allow the widest perspective for viewing the building, watching the people, good visibility to the stage and ample listening opportunity. We suggest you dress for the occasion (long pants, nice shirt, do not really need a coat, but I often wear one; a dress or nice skirt and blouse for women). We take photos after the concert. One concert was half full so we moved seats without any comment from the ushers.
We also encourage you to take a tour of the place prior to attending the concert. There is an informative video on the history and a guided tour. Recall the place is air conditioned so we encourage folks to schedule it into an afternoon as an escape from the heat. There is a tapas bar, but it is pretty dog gone expensive. Enjoy!