Late response but I wanted to add that your observation is the same thing I ran into years ago when first getting good subs to integrate into my system.
Many people that haven't done this exercise or who don't have home theater experience will try to predict crossover points based on speaker capability (measurements). It may look great on paper to have a higher crossover point. But in practice 80 Hz is the highest you'll be able to go before getting into territory where certain voices will start to sound chesty in movies because some of those sounds are being routed to the subs through the AVR's bass management, especially if you have the subs boosted with a house curve or loudness type feature. You'll find you get the best clarity by lowering your XO to the 60-80 Hz area. In addition, you'll get a better sounding blend between the speakers and subs to the point where it is seamless. You start to lose that above 80 Hz in my experience because the rolloff of the subs is not fast enough. Many people don't realize that if you're only rolling off the subs at 12 dB / octave, there can be a significant drone which you don't want.
So while 100 Hz crossovers are possible, it's pushing the limits, and I wouldn't go there without the ability to steepen the filters.
This is based on my experience over the last 10 years with powerful, quality subs (Rythmik).
