The increase in SPL by increasing the voltage by 3 dB will always be less than 3 dB due to the speaker not being 100% efficient (it has losses).If the speaker is being operated at volume levels that are not high enough to cause it to behave in a significantly non-linear manner (e.g., for "thermal compression" to become significant), then 3 db more in will result in very close to 3 db more out, as the % efficiency will be the same at both power levels.
On the other hand, see this post by Duke of Audiokinesis, about thermal compression.
Regarding Bombaywalla's comment about 50W vs. 100W, I believe he is simply referring to the fact that different 100W amps may differ from each other, both subjectively and objectively, in terms of their maximum volume capability with a given speaker load. Same goes for two different 50W amps. Obviously there are many possible reasons for that, including the ones he mentioned as well as the distortion characteristics of the particular amps at various power levels, differences in dynamic headroom, etc.
As far as the OP's question is concerned, yes amplifiers can have "sweet spots," but as others have mentioned there are a lot of other variables that may be involved, including the behavior of the speakers at different volume levels, the Fletcher-Munson effect, amplifier-speaker interactions, and acoustical differences between your listening room and the shop at which you listened to the speakers. Or perhaps the problem is simply that capacitors in the Aragon have degraded over time, and need to be replaced. I don't see a path forward at this point that wouldn't be somewhat hit or miss.
Regards,
-- Al