If your site passes WAI-A validation, Bobby, etc. then there is a good chance that you've covered SEO at the advanced level.
We do WAI-AAA these days. I haven't tested with Bobby in some time. It was having real problems, and WebXact kept trying to sell me their pro product whenever I whined about their sick baby. I gave up when it became clear they had no intention of ever updating Bobby.
Someone on this forum told me about Total Validator, and I also test with Hermish and Cynthia Says, so I try to get everything in I can.