Don't get hung up on the 85db number. Most speakers fall between 85 and 88. The horns have 90+ but that is another niche.
Mine are 87db and deep to 3.8Ohms, and the ST-10 could punch a hole in them with ease. Go for two STA-9 if you want the warmer sound, go for the ST-10 for more neutral sound. Don't worry about power.
You could even go with a single STA-9 and see if you are missing power (I bet you won't) and if you are, go with those second one.
To clarify, no, no one said that one 9 has more punch than two, he recommended two 9s (in modo mode) or two 10Ms.