I have a Pioneer DEH 150MP head unit and I've had it for a year. This is the first head unit I've had that had some subwoofer crossover and high pass crossover built in. The max volume goes to 62 and I usually have it around 45-57 with music. The subwoofer level adjustments go from +6 to -24. It also has a low pass crossover with either a -6 or -12 slope. It's low pass crossover frequencies to choose from are 50hz, 63hz, 80hz, 100hz, 125hz, 160hz, 200hz.
I know some people use their head units subwoofer crossover since it's easier to adjust. So would I use a 0 db, or -5 db test tone with the low pass crossover at 80hz on the head unit and set my amp gain with that with the sub level +6 out of -24 or set it at 0?