MOSFET H-Bridge Amplifier Unbalanced Gain
You are taking the U3 drive signal from the wrong place.



Figure 1. Marked up schematic.

Instead of taking the signal from (2) you should be taking it from (3). 

The problem is that (2) is a distorted signal as it has to overcome all the distortion caused by D2 / D3 and M1 / M2. You are tapping off part way inside the feedback loop of U2.

Your other option is to take it from (1) but configure U3 as non-inverting amplifier with stage gain the same as left side.