Unfortunately ratios aren't how logarithmic scales work and it's deceptive to frame the explanation in such a way.
I typically listen at an average power level less than 3 watts. That gets me up into the 85-90dB region at my chair pretty easily. 6dB is perceived as a doubling in volume. The 60 watt class A envelope allows for 13dB of headroom beyond that. It'll drive 120 watts in class AB, but that doubling of power is only buying another 3dB. 400 watts? That's over 111dB. That's extremely loud even for transient peaks. That will damage hearing quickly. I personally have no desire to own an amp with that capability. It's pointless, but beyond that, it's becomes more technically challenging to build an amp that performs excellently at both 2 watts and 400. I'd much rather have 2 or 3 truly excellent watts than 400 dangerous, very seldom uses watts. But, if you're using very inefficient speakers with ugly impedance characteristics, you'll need that kind of power. Maybe more.
I typically listen at an average power level less than 3 watts. That gets me up into the 85-90dB region at my chair pretty easily. 6dB is perceived as a doubling in volume. The 60 watt class A envelope allows for 13dB of headroom beyond that. It'll drive 120 watts in class AB, but that doubling of power is only buying another 3dB. 400 watts? That's over 111dB. That's extremely loud even for transient peaks. That will damage hearing quickly. I personally have no desire to own an amp with that capability. It's pointless, but beyond that, it's becomes more technically challenging to build an amp that performs excellently at both 2 watts and 400. I'd much rather have 2 or 3 truly excellent watts than 400 dangerous, very seldom uses watts. But, if you're using very inefficient speakers with ugly impedance characteristics, you'll need that kind of power. Maybe more.