While it's better to do what akirby suggests, you can do this with a "sequence" - I think this works with a 555 using the latest software?

Go into your "watch TV" activity and customise buttons. "Add sequence" allows you to assign a string of IR commands to a single button - create a sequence that turns on your amp and sets it to the correct input.

Only problem with this is, I don't think that the "smart state" logic will track the fact that the amp is now switched on. If you use this button, then switch to an activity that uses the amp, it'll probably get turned off, unless your amp has discrete on/off commands.