My best guess going from the description on their website as it being:
"Modern, High-Gain/Resonant NFL"
IV – Setting IV brings the NFL back in and adds a low frequency
resonance that pairs quite well with high gain voicing applied to
the Channel Controls.
Unfortunately, that description above is woefully vague!
According to this thread, it's the Mesa Boogie Double Rectifier:
In that thread they discuss the impact of changing the cabinet models, and the output modes, which may be playing a factor in how you are hearing the difference between the HD500 and the DT by itself.
In terms of the L6 Link connection, I would guess that when the HD500 is L6linked to the DT25 head, it defaults to stack/poweramp - though I don't know if that also matches what you would hear from the DT by itself.