Line Driver A: Max output 10v
Amp B: Max input voltage 4v
HU C: Max output voltage 4v
Line Driver A with master volume control has 10v output. Amp B has max input voltage of 4v. As soon as input into Amp B exceeds 4v, it is clipping, much earlier than 90% of full volume.
If using HU C, you will not be able to drive the amp into clipping as the max voltage output cannot exceed the max input.
This will result in the same performance, except that the line driver would produce the max unclipped power at a lower volume control setting. The signal itself is no stronger.
