The bottleneck of these units has always been the analog stages, so there is no point in a converter swap without addressing those first. (They are riddled with crummy ceramic capacitors in the signal path and NJM2068s subjected to high levels in unbalanced circuitry.) Once those are modified, you can basically get datasheet-level performance out of the AK4393, about as good as anything remotely affordable in the mid-2000s. DAC SNR of 117.5 dB and THD of -105 dB is more than good enough for a crossover, you can clear a 110 dB system dynamic range and any human distortion audibility threshold like that.Also, do you think it is possible a modding of the unit by replacing the AK4393 with a more modern DAC via I2S?
Using a more modern DAC would require some glue logic to properly accommodate the pins used for speed setting and such.
I bet they kept using these 96 kHz parts because they were actually top-notch converters that were considered outdated within just a few years (with 192 kHz being the new hotness) and may have sold at quite a discount as a result.