So the FR response of your towers is rated to 45hz. With a little in room bump you should be fine at 80hz, however your subwoofer isn't the most capable one ever so if it were me I'd play around with it, but lower it a little to maybe 60. I think 40 would be too low, but 120 is definitely too high. With a more capable subwoofer I think you'd be happier with 80, but given the sub, I think 60 may end up being more appropriate. Then again, you may not notice any difference at all.