Speaker impedance curves are rarely flat - a speaker may have a 10 ohm impedance at 1 frequency, and 4 ohms at another, both within the range of frequencies the speaker plays well. If voltage were constant regardless of frequency and controlled by the knob, wattage would be much higher at 4 ohms than 10 ohms, and I would imagine volume would be much higher too. Similarly (but the opposite) applies to current. If the amp holds wattage steady, and is 220 into 4 ohms, what happens if I play the frequency with 4 ohm resistance, turn the volume all the way up, and then play the frequency with 10 ohms?

I figured someone on here actually builds amps and would know.