Keep in mind orange peel and urethane wave are two diferent things.
Peel can be rid of with fine grits like 1000 to 2000 paper.
But it won't get rid of the wave, it'll be smooth but not straight.
To get it straight needs the coarser grits.
It's more due to the grit selection than the block.
You won't see much difference from a hard block when useing the fine grits.
They don't straighten, they smooth
It's also important in how you sand to get it straight.
Long strokes from edge to edge of panels and/or body lines.
Following through with your strokes.
Short back and forth strokes can show as lines.
To get one truely straight takes some pretty aggressive sanding,
way past just the leveling of orange peel.
And of course, your primer has to already be straight to start with.