There are two issues here. First, a stock 1056B03 doesn't accept EFCs (keymoves). Second, it uses 4 digit setup codes rather than 5 digit ones. Instead of entering 21781, enter 1781 while in AUX mode. (the leading 2 would tell a RCRP05B that you want an AUX group setup code.)
But since the 1056B03 doesn't accept keymoves, setting up 1781 isn't going to control your amp. Many UEI remotes do accept keymoves using the 994 command, but they may or may not have the 1781 setup code.
If you buy a <$10 USB to RS-232 cable [see
[Link: hifi-remote.com] read all the way to the end] then you can fully program the 1056B03 or JP1.3 or newer class remotes using
RMIR. I recommend that website for anything involving UEI remotes.