Greg, an 80Hz crossover is suggested by THX and others as being a reasonable compromise; high enough to take a significant part of the lowest bass burden off the other speakers and the amps driving them, but not so high as to make the sub easily localizable. Unless your sub is located far away from your mains and doesn't blend in well with them it probably makes little difference to use a 100Hz crossover and wouldn't be a good reason to buy new equipment.
Attempting to use the internal sub crossover isn't helpful in this regard. If the speakers were still set small, the 100Hz receiver crossover would still control both the low pass to the sub and the high pass to the speakers. Use of the sub's internal crossover affects only the low pass to the sub(possibly interfering with the receiver in this function), and doesn't have any effect on the high pass to the speakers. If the speakers are set large, there is no crossover involving them(again, the sub's crossover doesn't affect them), they have to play full-range, and the benefits of bass management are lost.
