I have the same headphone volume problem but I forgot all about the monitor boost feature. If for some reason that doesn’t work, enough for you, do what I did, and get a small headphone amp, or any kind of headphone amp. I actually bought the bigger 6 channel one from ART and it works well, plus I can use the other channels on it for other headphone mixes.
Well, I finally tried the “monitor boost” that Lazlo talked about and it works pretty good. I haven’t used it for a live “loud” recording yet, but still, again, if that isn’t enough for you, use some kind of headphone amp. I know for me, even before I boosted the monitor volume, once I ran the stereo output through my headphone amp, that seemed to do the trick. Marc Trainor