I was at someone's house today and saw that their DSL modem was synced at 400k/160k. I had a look around to make sure that all of the phones were filtered when I discovered that she had a filter on the main phone jack in the house off of which everything else, including the modem, branched. I removed that filter and filtered a branch with a phone on it, and the modem's downstream synced at 860k. I always thought that DSL filters would block the modem from syncing at all. I've never actually tested it though.