As @nsxtasy stated, there really aren't any dress code spots in Madison. L'Etoile is bar none the most heralded restaurant in the city, if not the state. If you're looking for culinary gravitas, go there if you can.
Sardine is quite nice, with a romantic interior--especially if you get a seat near the lakeview windows. Restaurant Magnus has a dark, sultry vibe, although now that they feature a modern Scandinavian menu, the passion is maybe a little reduced from its South American fusion days.
Brasserie V's menu on Sundays is a "lite" menu, so you'll be missing out on its best main courses; the new late-winter menu just debuted on Monday.
Johnny Delmonico's is a straight-up steakhouse that does a lot of Valentine's Day business. Consider Inka Heritage if your desire for a dinner date can accommodate the unconventional.
Do you mean Madison, Wisconsin? If so, I'm sure the Chowhound Team will be happy to move this topic to the Great Lakes forum.
You'll want to take a look at the existing topics for Madison:
By "dress code", if you're referring to requiring jackets for gentlemen, there isn't any place in Madison that does so; business casual attire (shirts with collars, slacks other than blue jeans, no jackets, no ties) predominates even at the nicest places, like L'Etoile and Harvest. You'll need to check with the individual restaurants or their websites about their plans that day, though, because many of them are closed on Sundays.