Montreal is a wonderful city, though some items on your wishlist might be a bit tricky.

For a big old romantic hotel, you won't do much better than the Ritz-Carlton on Sherbrooke St. Lovely old place, nice dining room, in a good location near art galleries/shopping. A distant second choice for an old romantic hotel is the Queen Elizabeth (now in the Fairmont chain). It's mostly a business hotel, excellent access to the underground shopping center (more important in winter than the rest of the year).

One of the nicest parts of Montreal is the old city, centered around Place Jacques Cartier. Lots of interesting places to stroll around, shops, street artists, restaurants.

Theatre may be very tricky unless you're francophone. Although you'll find English spoken in just about every shop in Montreal, French is the native language of most residents, and way more theatre is in French than English. However, Montreal is also the home to the Cirque du Soleil, whose shows know no language. Check their home page for schedule.