I don't have the equipment to test actual frequencies, but I do think I am getting at least full lower frequencies. When I was tuning my amp I started with only the lowest setting i think around 20 Hertz? And it was still hitting. Since it's a 10 I have it catching somewhere around and between 20 and 110 hertz.
As far as I could I know a deck usually will put out a full frequency and low frequency for a sub. If your friend has the premium sound, and tapped into a line after it passed through an amp or on the line to his sub, then there is no telling what sound range he will get. The premium system apparently has a couple amps and another thread covers it well. I think tapping in right behind the deck has given me the full frequency.
Sent from my Nexus 5X using Tapatalk