The lower you go, the more "up front" and transparent your bass will be. However, the vast majority of 6.5 woofers can't take lower than 80Hz. (I mean, they will still play 60 and 40Hz test tones just fine, but there is a big drop in the level and increase in distortion). I would start at 80Hz. I always high passed my front speakers (Alpine Type R, Image Dynamics Imagine, Infinity Reference, Pioneers, etc) at 60Hz and never was happy about it. I blamed the subwoofer, records, etc. Then I moved to 80Hz, and the music got a lot smoother to my years (read: less distortion).