I'd keep doing both....keep checking on line and keep calling to see if there's a cancellation. I don't think they hold tables for a party night, but I don't think they'll let you make ADR's after a certain time if you don't have a party ticket for that night.
But people always cancel reservations, so it's definately worth continuing to try !! Don't give up!! also, try putting different time increments in....sometimes if you put an hour earlier or an hour later, something good will pop up for available. we found that with Flying Fish.....I wanted to get 6:00 pm and nothing came up when I put in 6:00. When I put 5:00 in, 6:05 showed up. Weird, but I grabbed it !