I [incorrectly] made "mejor" plural - to agree with "they" - by writing: "Eran aún mejores que en mis sueños". [Or could it also be correct with "mejores"?]
Perhaps we have to regard "mejor" as an adverb here, not as an adjective - so we should not make it plural - despite the fact that (in English) it looks like a complement of the verb "eran".
My grammar book (by Butt and Benjamin) seems to confirm that^ by giving the example "Aquí estamos mejor" = "We're better [off] here".
It seems that we need to be careful in deciding whether a particular word in Spanish should be treated as an adverb rather than an adjective. In particular, we should avoid the temptation of trying to judge it according to its grammatical context in English. [A well known example of that, is of course "Está bien"].
