80 Hz is a good point because it's around there that we start losing localisation making the blending with the main spkrs easier. Also the high frequency sound fm a 12' subwoof driver is not linear (that driver wasn't made to tackle midbass).
As to the dip, don't worry -- try it: crossing over at, say, 80Hz means that you ATTENUATE the sound 80Hz (+upwards) from the subwoof so there is overlap with yr main spkrs. If the blend is good, keep it.
Finally, the 12' if you want to afford it is a better bet for low frequency reproduction -- its displacement volume is better than the 10'.
Generally, Velodyne makes nice user-friendly subwoofs.
As to the dip, don't worry -- try it: crossing over at, say, 80Hz means that you ATTENUATE the sound 80Hz (+upwards) from the subwoof so there is overlap with yr main spkrs. If the blend is good, keep it.
Finally, the 12' if you want to afford it is a better bet for low frequency reproduction -- its displacement volume is better than the 10'.
Generally, Velodyne makes nice user-friendly subwoofs.