The percentage volume scale is linear. It's not a good way of representing the change in volume.
Here's a quick sketch to illustrate the relationship between the linear volume scale and what's happening with amplifier power. I've taken a little artistic license with the figures to illustrate the point.
Volume and the way the volume control works is better represented by the red curve - it's logarithmic. A 6dB increase means twice the amount of power being used from the amp. At the bottom end of the volume range the power is still doubling, but it's doubling very small amounts. The linear percentage scale shows big changes; you could go from 0 to 50% and not hear much change. As the red curve starts to tilt up then the effect of doubling the power becomes more significant. But it's only in the last 20-25% of the linear scale range where our ears perceive the biggest changes. The closer to 100% the greater the volume increase. So once again, the linear percentage scale is a poor indicator. The last 5% of the linear scale might account for 90% of the amplifier's power range.
Having an amp that seems relatively quiet at 75% of the linear range isn't such a big problem. Most of the power is yet to be delivered.
There are lots of factors affecting volume (or, more accurately, sound pressure level), but some arbitrary percentage volume bar isn't really one of them