The one room I have used these in did not have huge reflectivity issues with the 80x80 pattern, but it has a 12 or 14' ceiling so no idea on the 10' ceiling in real life. It was a long and narrow hall and I actually used it 1 over 1 with a TH 115 on the bottom.
The SM80 really drops off below 100 Hz. I have the subs crossed around 90 Hz w/ 24dB/oct and the tops around 110 Hz with 12dB/oct, and don't really hear a hole at 100...I typically don't like a lot of excess energy at that frequency anyway.
The thing that a lot of people miss-is the difference between the ACOUSTICAL and ELECTRICAL points in a system.
Electrically it may seem like there is a hole-but that would only be the case if the loudspeakers on either "side" of the crossover point were flat in response and run at the same level.
This is RARELY the case. For the case in point (top and sub) most subs are louder in the end of their response than down at the bottom (so louder at 100-200Hz than at 40-50Hz).
AND most people run the subs hotter than the full range signals. So the ACOUSTICAL crossover (basically where the subs and tops meet) is going to be higher than either the low pass on the woofers or the highpass on the tops.
The same thing happens going from mids to HF devices-except the opposite way. Most HF drivers get really loud down towards the bottom end of their response. So that extra energy has to be "done away with", by crossing over higher-so the ACOUSTICAL output is now more even.
Now sometimes this "excess energy" is delt with by using EQ, other times by shifting crossover points-it really depends on the particular situation and the final phase/amplitude result desired. Sometimes one approach is better than another-it depends.
So if there is a "hole" electrically", it will (in many cases) be filled in (acoustically) because the level of the sub is so much higher.
This is true of most loudspeakers.
The END result is what we are interested in (what our ears hear) not what the "numbers are".
I get asked all the time "where are the crossover points in a particular box". What does it matter-if you can't see it on a measured response or hear it-WHO CARES. Does it matter if the highs are crossed over at 500Hz or 2Khz or 8Kz? If you can't hear it-then it is of no concern-except that some people want to "judge" a design (that htey may not fully understand what is going on) based on some preconceived notion they may have-which in many cases are wrong.