Question about volume control on Plate Amp.

Hello I have a question about the volume control on the plate amp. Its rated 40x2 for 4 ohm but i would like to use it with some 30w drivers. The thing is that I would not want to accidentally overpower the speakers with the volume control on my phone. So my question is will the volume knob limit the maximum volume that I can control with my phone or how does that work?