I have owned both of these heads at the same time for a good while. I have my 400 loaded with Tung Sol 6550s and a low gain/high volume tube in V1 (forget what, a JAN something). D180 with EQ has Mesa power tubes and a JJ in V1. I use about the same settings on each (Mid 8-10, Bass 2, Treble 2). 400- Set more a less "flat" (using settings above) its kind of hard boxy midrange is the core voicing. D180- Same settings, a little "rounder" in the midrange. Overall I find the D180 to be a little bit more articulate when kept clean (to test this I ran my 400 Master about maxed, which is how I run the D180 for clean), with some of the finer dynamics of my playing cutting through more clearly on the same cab. Could be the 6550s being all beefy on the 400. The 400 appears to be a little bit spongier (even though I use the equivalent preamp channel on both), a dash less clear on finger style details, and a little more colored (which works great for me because when I play my metals aggressively, like 8-10 on an aggression scale, this slight "cushion" really forgives my playing while allowing the snarling mids I like to come through). For those who have ever wondered, here is a cheap video I made of how the distortion on the D180 sounds with guitar (drop B). The cab is a big part of it though, it sounds way more 80's on other cabs.