i generally set my subsonic a few hz below tuning. my livingroom HT settup is tuned to ~17hz(dont ask, please god dont ask) and the filter is set at 13hz. in the bedroom tuning is ~25hz and filter is set to 23. the car is tuned to 30hz and i set subsonic at 29. depending on the sub and power you can go anywhere from 10hz to 3hz below. but remember, below tuning bad **** can happen fast. lol
Matt