Unless you're attending a season opening, production premier or a gala performance, I think that "dressy business casual" up to 'jacket & tie' should be just fine.
One way you can get an actual idea as to dres is to look at the audiences shown when an opera production is presented on DVD. For example, during th production of 'Il Viaggio a Rheims' at the Liceu in Barcelona, there are plenty of shots of the audience, as the singers often go into the auditorium for various arias and ensembles. The dress of the people in the Luceu audience the night that the DVD was taped exactly fit the description above (IIRC it was taped in either late 2003 or early 20040.