I am curious as to the physical differences in the old/new wires. What is the gauge of the two wires in question?

Nobody has ever disputed that a wide disparity in wire gauge over a distance can cause a voltage drop, which is measurable and can, in some cases, be audiable.

If that's the case, then there could be a reason for you having to drive the amp so hard as to strain it, which is what you seem to describe. Using a high gauge (thin) wire for a long run is a no no.