For 75-85 dB a few watts are enough so ...

I agree with Meiwan and Ddd1. Speakers that are not efficient take an amp with a lot of current. A 84db speaker will require 2-4 watts (depending on room size) to play at 84db from the listoning position.
Most amps with 10-20 watts do not have the current to drive the hard load that the 84db 4ohm speaker requires to sound full and dynamic. It will reach the SP's but lack in punch and dynamics.
Hope this helps.

John

You also have to keep in mind that a low powered amp pushing a hard load can clip because of the lack of head room. You may not even know your destroying the speaker beacause of soft clipping.