I've heard The Horns with a 2A3 based Fi X amp and a 1.8 wpc Decware Select. Either played loud enough for most of us in a 3,600 cu. ft. room (a made over 2 car garage). Most seriously listen around 80 dB. Note that extended exposure to more than 85 dB will result in permanent hearing loss. If you're not sure how loud is loud get a hold of a sound pressure level meter.
Note that this driver is only rated to handle 24 watts anyway. With 2 wpc you can typically expect 93 dB from The Horn and 103 dB from 24 watts.