Accuracy when measuring THD with LT Spice for AB Audio Amp
Given the open loop gain of the OPA197 is over 1,000,000X you might indeed have only 5ppm distortionAT LOW FREQUENCIES. This ignores any gain of the bipolar current_input discrete power amplifier.
With UGBW of 10MHz, the opamp alone should provide 10 ppm at 1,000Hz into a light load.
For excitement, examine the transient linearity and settling of you current mirrors. Use a 1us input risetime, and then a falltime.