Audio Slave;
6P3P is a chinese tube Beam Tetrode can give 4.5w at 250 v, same pin base connection as 6v6 0r 6l6 but lower rating. 6N8P is a dual triode tube for preamp and drver applications, same pin base connection as 6SN7. You can experiment, but the characteristics curve are not exactly the same, but I think an amp with this tube can also use the 6sn7.
KT66 similar to 6L6 but not exactly the same, but will work in most 6L6 amps same with5881, but 5881 will not be always suitable to 6l6 amps, it will depend on the Plate Volrage used in the ^6l6 amp.
I hope I did not confuse you.