So far as I can tell, it's really difficult to tell the difference. I've tried many different song types and bit rates (24/48, 24/88, 24/96, 24/192).
The most noticeable difference is the treble decay and fall off maybe. Also soundstage, the HM-901 on 1.084 is maybe slightly more open sounding? Slightly less congested? It's so subtle.
There may be a tiny bit of high Bass, low Midrange emphasis, bump up. Maybe that part is what makes it sound warmer perhaps?
I don't know, it's kind of too close to call, because the playback behavior is quite different now, because 1.084 cuts off a lot more of the beginning of songs. 1.085b still cuts off the beginning, but much less so.
Maybe there's sample variation between the player builds? My HM-901 is one of the first ones made, and the other 2 I'm comparing are more recently built.
And I couldn't tell a difference really with Digital Bypass Filter being On or Off. I'm exactly sure what it's supposed to be doing. Something with the Dock?