I have had both the 170's and 340's as mains XO 80hz. Yes, the 340's are noticably better because they can play louder with less distortion. I would say if you never get above 85db or so, the 170's would more than enough. However, if you are like me and really get on them, you would be happier with the 340s
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