You will need to find speakers that are not only efficient, but also easy to drive. This means a true 8 ohm, or higher, impedance speaker. Tekton speakers mentioned above might work and they sound pretty good to me. Zu is also quite efficient, but don’t suit my taste—they are too harsh and brittle sounding to me.
Although they are not extremely efficient, Audio Note speakers are easy loads and work very well with low-powered tube amps. They are meant for placement near room corners, which boosts their practical efficiency—they sound much louder than their efficiency rating would suggest. The AN-E model might actually have too much bass for a room your size, but it is more efficient than the smaller AN-J model which is meant for a smaller room.