That's about right John.
Now, let's figure it using just math that most use.
The Klipschorn is 104db/w, add 3db for the other speaker, and we are sitting at 107db/w.
1 watt = 107db
2 watts = 110db
4 watts = 113db
8 watts = 117db
You are about 12' back, and so now we subtract 3db for every 3 feet you move away from the speakers.
117db at 3 feet
114db at 6 feet
111db at 9 feet
108db at 12 feet
So now, someone would say that with 8 watts and Klipschorns, you should be able to hit 108db, 12 feet away -- but that's not what you found is it? In fact, you found EXACTLY what we have been saying since last month: That you need between 3 and 5 times the steady state amount (depending on recording), to reproduce the peaks cleanly. Your ears were telling you that 99db was pretty much IT. Where did the rest of the ouput go? This is the difference between "watts", and "acoustic watts".
Now, and I know this is hard for people to accept, but because of the soft clipping nature of tubes, and especially SET amps -- dynamic peak restriction is clearly evident when you compare lower power to higher power. For example: with a 30 watt amp, your steady 99db would have been generating peaks of near 104db. Try your experiment again with another amp. Take it up to 99db and watch the peaks.