Are you using a computer as the audio source? If so, you could use your computer as the main volume control.
Turn the volume control on your computer to 100%. Then turn up the volume on your amp as loud as you want your speakers to go (just as long as they are not distorting). Then adjust the sub to match at the same time. From then on, user your main computer volume control.
I havent decided if I should use my computer, or just leave my Z2 connected to the amp... But my main concern is that I do not know how to connect the sub to the amp, as there is no subwoofer outlet. I found some info online, but I dont really understand what it means
Sorry. I was typing my response fast and forgot to say.
You should be able to use the Rec Out for one of the Tape options. It's a line out meant to allow recording with a tape player. So it won't vary with the volume control on the receiver, which is why you have to use some source plugged into the receiver that has a volume. Plug your computer or iPod or whatever into Aux or a different input from the Tape use choose, and it should send the signal output to the Rec Out by using the Rec Out Selector on the front.