Both the DP88 and D8 use XMAX preamps. The DP88 utilizes digital control over the analog circuit so that each preamp's trim, phantom power, and direct-to-ADC switch can be remote controlled via MIDI or over ADAT when connected to a Studio 192. This configuration also removes some of the inherent noise produced by an analog potentiometer. The DP88 also provides individual phantom power for each preamp, while the D8 provides phantom power for each pair of preamps.
As you mentioned, the converters are different in each model. The DP88 converters provide 118 dB of dynamic range and up to 96 kHz conversion. The D8 converters provide 107 dB of dynamic range and up to 48 kHz conversion.