One point that I think is important is that over the decades, and with experiments designed to test non linear
parts and designs in my test audio system, auditioners have found that the more accurate the parts and design, the better synergy, the more satisfied they were with the system/musical reproduction.
Using non linear parts/components and attempting to compensate in the speakers and other components always produced less satisfaction than when the accurate parts/components were introduced and attempted to optimize. This makes sense since overall synergy of the accurate audio system is maximized and weakness/"faults" were minimized.
Thus the OPT must be designed for maximal flatness in frequency response, actually in tonal balance, just like any other part. We have already seen over the previous pages the problems with high frequency response and to a lessor degree to the low frequency response. Rectangular waveforms can be useful in exposing problems.
An OPT that is bass heavy/bass light or high heavy/lacking in highs will require compensation of the rest of the circuit in order to "compensate" as best as possible. However, as stated earlier, optimum satisfaction with auditioners occurred when all the parts and components were optimized for fewest "faults", thus the best synergy.
The only possible way to know for absolute sure if an OPT is optimal is to test the OPT in an optimally designed circuit. Easy to state, difficult and time consuming to accomplish.
Cheers
Steve
