Back in March 2001, Car Stereo Review published a review of the Empeg MK2: http://www.m-emag.com/reviews/reviews.html?reviewID=60
Frequency response of the unit was measured at -2.4dB at 20kHz, which you might argue is not audible, but honestly is pretty lousy for a digital source. This is consistent with the Empeg spec sheet, which gives the freq resp of the aux input at 20Hz-18kHz. Maybe they measured an older unit? However, most sources sampled at 44.1kHz (like a CD) suffer from horrible quantization noise above 20kHz, and you'd need a brick wall filter to get rid of those artifacts -- I'm not sure I'd agree that freq resp would be flat past 20kHz. Most mp3-encoding methods get rid of that freq range anyway because it's not audible and it's a waste to encode. In fact, the latest r3mix method for LAME has a low-pass filter at 19.5kHz.

The channel separation was also measured by Car Stereo Review: 40.1dB at 1kHz and 38.2dB at 20kHz. I think a record player has better stereo separation -- again, pathetic specs for a digital source, and i would argue audible in a well set-up car system. This is consistent with the 50dB stereo separation published in the manufacturer's spec sheet for the aux input. I'm hoping that the poor separation is from the analog output stage....if it's from a stage before the proposed digital output (aka in the DSP equalization stage), then the outboard DAC ain't gonna do squat. I'm not sure where your 75dB value is derived from.

If there are any other lab measurements available, I'd definitely be interested!!! The slight loss in high freqs I could care less about, but 40dB stereo separation is horrendous. Thanks!!

Valsalva