Hi Voldemort,
I'm not sure that it's possible to do what you want via Bluetooth - it all depends on the Bluetooth Profiles that each device supports, the direction in which the audio's being transmitted, and what the receiving device expects. In the scenario you describe I suspect that your tablet is probably handling the incoming audio from the amplifier as though it's from a hands-free phone microphone, which of course isn't what you want.
More info:
https://en.wikipedia.org/wiki/Bluetooth#Bluetooth_profile
https://en.wikipedia.org/wiki/List_of_Bluetooth_profiles
There are several threads elsewhere on this forum about the Raspberry Pi's Bluetooth feature - just do a search.
I hope this helps,
Peace and friendship,
englishtim