I have my Ascend 170se crossed over at 80hz to my VTF-3 HO. I also used 80hz for my 340se.
Some people don't realize that just because your speakers can play down to 50hz, doesn't mean that's where you should cross them over. A good subwoofer like the HSU will be able to play 60hz at higher volumes with much lower distortion than any bookshelf speakers, even the 340se.
p.s. I just added my BFD EQ to my VTF-3 HO and it is sounding much better. I was able to reduce a peak around 40hz as well as boost the 70-80hz bass giving me more of that mid bass punch.