Is the Audio research ref75se enough power for the sopra no2?

[sonicmatter]

Hi guys,
I'm using arc ref75se driving sonus faber olympica lll for a year now and noticed that the amp is not efficient enough for the O3's. At high volume the O3's sound strained and harsh. The amp is 75wpc push pull tube class ab and the O3's nominal impedance is 4ohm and sensitivity is 90db claimed but actually is 88.8db measured by hifi news mag. Is this because 4ohm speakers are hard to drive then an 8ohm speakers? Room size is 15' x 15' only.

I'm thinking of changing it to the sopra no2, which is 8ohm and 91db claimed but actually is 88.7db mesured by hifi news mag. My worry is would the arc ref75se have enough power for the sopra no2?