@ulul 18 Watts (or 30 into 4 Ohms) isn't a great deal of power. From the looks of the distortion curve vs power, if you want the most out of this amp it really should be loafing. I think it will sound fine with any of the speakers you mention, but that won't be the same as sounding its best.
It seems this amp flies in the face of most amps by having actually lower distortion into 4 Ohms, while still having a good harmonic spectrum of the distortion.
The trick is that speakers are measured using 2.83 Volts at 1 meter. So if its a 4 Ohm load, to make 2.83V Volts the amp is making 2 Watts. So a speaker that is 95 dB and 8 Ohms is 3dB more efficient than one that is 95dB and 4 Ohms. For that reason you are looking for a speaker that might be 100 to 103dB and also 4 Ohms, which strikes me as a bit unusual.
In an average room I found with speakers of 97dB that I needed a lot more power than I would have guessed, so as to keep things relaxed at normal listening levels. Good luck with the quest!