An amplifier won't help you just cost you money and make your wifi work worse from increasing your noise factor.
TP-LINK uses 5dbi antenna's you won't notice any huge improvement with aftermarket antennas.
You should look to placing your router in a central location in your house.
If you have a 2 story place it on the main floor where the waves will have the most clear propagation to your stairs for both your basement and upstairs.
