I’m assuming your preamp has a resistive volume control. If that’s the case. the "higher" the volume setting the less resistance.
"Less resistance" where?
You don’t know where the volume control (VC) is located. Is it the first device in the chain functionally a load for the source, or is there an input buffer then the VC, or is it on the output of the preamp which would be very unusual. In any of those scenarios, the total resistance seen by the previous stage will include whatever is loading the VC, which may be so high compared to the VC that the total resistance is effectively constant.
The output voltage of the preamp should drive the amplifier to maximum power output at just about the full volume setting. That’s when the attenuator is resisting the least and converting as little as possible of the voltage gain to heat.
This assumes the VC is at the output of the preamp. This is extremely unusual. It is also inconsequential if it is. The output impedance of the pre will be much lower than the input impedance of the amp so the load it presents will be effectively constant no matter the volume. The VC will dissipate the same amount of heat (extremely little) whatever the volume so it simply doesn’t matter.
The bottom line is without knowing the topology of the preamp you simply have no idea how the VC is affecting anything. . In any case, it doesn’t work as you described.