Nice summary on a fantastic card, Telix.
For me, the LM6172 op-amp's were great for my Sennheiser HD650, they were very bright and energetic which was great for livening up the senns. I would say that the LM6172's had a smaller soundstage which was also a good match for the HD650s as the soundstage was already quite large on them.
Now that I'm using my DT990's though, I've switched to the LME49720 op-amps to counter-act the DT990's smaller soundstage and already prominent highs. Seems like a good pairing so far.