5 - 20 watts is the ideal range for direct connection to a highly efficient horn. Any more than that is utterly wasted (I'd argue that more than a couple watts is wasted), and there is increased risk of blowing out the diaphragm either unintentionally or by electrical fault (a series cap won't help with too much power!). The power consumption issue I can easily see if we're talking a 50 watt single ended amp, but a low wattage single ended amp actually draws less power than a conventional class A/B home theater amp.
BTW, my HF horns (500Hz crossover) are 107dB efficient, just a tad more than yours. I can create
blisteringly loud sound while not even coming close to the limit of my 2.5 watt SET on the HF horn. The lower frequency horn handles below 500Hz and these are driven by a 40 watt amp which is coasting along the vast majority of the time, if not all the time. Where the real power output is needed is the subwoofers, and those get 1000 watts.