Hi,
For the sections "Eran aún mejores que..." and then after this "...en mis sueños.", I believe it might be a bit confusing to separate these parts of the same sentence as the use of the imperfect or preterite tense for the 'Ser' in this sentence relies on the context of to when this action refers. Just seeing the prompt "They were even better than...", I assumed that it was referring to the actual concert, which you would use the preterite of 'Ser' (Fueron) as it is a single and defined event, however when you see "...in my dreams." you then see it was referring to multiple, repeated instances so you would hence use the imperfect tense (Eran).
Perhaps I am mistaken?
Regardless, these quizzes are very good for practical and realistic Spanish usage.
Thanks,
Angus.