MOON gear is generally applauded for its ability to sound the same no matter what the output volume is, and the I66s can certainly play loud with no diminishment. So my guess is either the 240 output isn't enough or, and I think more likely, the hard room reflections are adding up as the volume increases and muddying the overall sound.
A cheap trial would be to temporarily modify the acoustics of the room with cushions, mattresses, wall hangings, more furniture, etc. and then turn it up and listen.
The 240 puts out 50w into 8 ohms and 85 into 4 and a phone call to VAF should get you some info about their recommendations, or PM @mikizee.
Good luck.