Jafant:
The mods were done by an Oregon-based company called Jena Labs. You may have heard of their cables. My player was done a long time ago, and a lot has happened in the interim, so I've forgotten a lot of the details of the mod, unfortunately. But I remember she cleared the signal path of a bunch of crap and did a lot of damping, among other things. She left the Marantz HDAM output stage in place, because she really likes that particular design. Some other modders strip it out and replace it with something fancier.
The result was really eye-opening. After many comparisons over the years, I think it can hang with some of the big boys. It's pretty good, IMHO.
Sorry I couldn't provide more technical info on the mods. The 'ol noggin' ain't what it used to be!