Here is a paradox concerning amplifiers. More speakers, both professional sound and home stereo, are damaged by having too little power than are damaged by having too much power! See 11:59 post about clipping. A good rule of thumb is that tube power amps should be capable of 2X the speaker's rated power, solid state amp 4X. That means, If I were asked to set up a sysem based around 40W speakers, I would get a power amp of (at least) 160W. You have an amp with too little power, not too much.

Where the volume knob is set is a function of "voltage gain", usually just referred to as "gain", and not a direct indicator of how much power is delivered to the speaker.