its not hard to change things and, well, change things. I remember switching from my 400W class D amp to a 400W class AB amp. sound was so much better controlled. Later, I switched from my 400W class AB amp to my 400W class D amp. again, the sound was more controlled with better impact. Moral of the story -- subjective observations are near worthless if you change any settings.
one of my favorite line driver reports went something like this: "so i was tuning my amp, but i couldn't turn it up all the way because of the neighbors. this week I installed my line driver and it made a huge difference!"
from an electronics perspective, the additional voltage gain on the input doens't affect the power supply. the power supply sets both the maximum output voltage and current, and thus sets the limit on power. so long as the amplifier has enough voltage gain, it will acheive this goal. Because most amplifiers can be driven into clipping with just the HU as an input, this would not be a concern. Also, because the amplifier itself has an efficiency that is dominated by the the output stage (class A,B,D), the small increase in voltage gain does not change efficiency.