Well, I always cross at 80hz and I just went from the 170SE's to 340SE mains and the difference was noticable right away. The 340's can definately play louder with less distortion than the 170's. The 170's are GREAT speakers, but the 340's are just better. Also, by crossing the 340's with the 80hz XO using a very capable sub, I don't think you lose anything. I think running the 340's as "large", a 60hz XO may possibly be better.
L/R - Sierra Towers
Center - Sierra Horizon Tower
Surrounds - 200SE
SW - Dual PSA XV15's
TV - Panasonic TC-P65VT60
A/V Receiver - Denon AVR-4311CI
Blu-ray/DVD - Sony BDP-S5100
DVR STB - Motorola Arris X1