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

My experience differs from Meiwan`s, a high quality low powered amp will more often then not sound much better at low and normal volume levels. Better in terms of resolution, inner detail,microdynamics,nuance and natural tone. I do agree that for inefficient, high current demanding speakers, high power amp are mandatory. A high efficiency/easy load designed speaker sounds superior with upper tier low power amplifiers. This type of pairing is very dynamic and IMO more natural and real/live in it`s presentation.

The largest cause of speaker failure is having an underpowered amplifier that clips and distorts loud sounds which can occur in music without you even being aware of them. This can occur even if you aren't listening at reference level.

Some need it to break their lease.

I think there's a lot of fuzzy thinking and bad math when it comes to this subject. 500 extra watts doesn't make the first watt any better or worse. 1 watt= 1 watt.