In theory the Classic is irrelevant as all Slingboxes use the same IR System. I'm also a bit surpised that none of other TIVO remoyes work, as all the TIVOs tend to use the same IR, with the exception of the TIVO button itself, which does vary.
Check out this thread: https://community.sling.com/thread/3300?tstart=0
Come back if it that doesn't help.
Later: I just checked and the TIVO Premier 4 is in the list of devices for my Classic.