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The Series 1 is the most hackable, but the series2 is much more capable. There have been major strides in the hackability of the series 2, such that you can now run TiVoWEB on it, and reportedly even run SA software on DTiVo hardware.

In the end, though I think you will find MUCH less success in hacking the TiVo than in your other exploits. The Xbox and Audrey are basically normal computers confined to do simple tasks. The TiVo is simple hardware - barely more than enough to get the designed job done. They don't have much left over resources, or access via the OS to do anything you would like.