I've been into mobile audio off and on since '87, have owned many dead head excellent sq HUs, and the x889 is the finest I've ever owned. Now let me qualify my statement by saying I've never owned an eclipse, or denon rf, or denon, but have owned all the top of line HUs from Alpine and Kenwood.
Just to be clear though, the 889 is not a dead head unit, I just brought that up, cuz most of the best SQ HUs of the '90s were dead head units. A couple great features of the x-889 are Burr Brown 24 bit D/A converters, excellent Digital sound processing capabilities including digital time alignment, 4 band parametric EQ, 5 volt pre outs, and the ability to turn off the internal amp when using external amps.