Interesting to me that some experience limits in regards to loudness with the OHMs.
I have not found this to be the case with either my 5s or 100s series 3 in their respective sized rooms at least, even off the 500w/ch Bel Canto ref1000m's I currently use, which push them up to levels beyond anything I would dare prior.
Due to their relative low efficiency, I have found the amp to be the limiting factor in regards to volume, not the OHMs. I listen to all music types from metal to big band to symphony as loud as I can go to get to what seems to match live performance levels with no signs of stress or breakup. The sound just keeps expanding with no noticeable duress until I finally stop at levels that seem lifelike for whatever happens to be playing.
OHMs do have essentially the same sound from model to model. Larger models and juicier amps are needed to get the most possible out of a larger room.
I would agree though that OHMs are very sensitive to room acoustics. Rooms that are exceedingly lively, like our sunroom with tile flooring, three walls of windows and cathedral ceiling, will sound much different and less balanced likely than similar sized, less lively rooms, like my office directly below for example. But then again,I hear similar effects with other speakers I use in the same rooms, including my small Dynaudio monitors. If I had to chose just one speaker, even in the lesser rooms, it would still be the OHMs. If I were willing to do extensive treatments in a lesser room in order to tame the acoustics completely, I would go with more traditional directional designs to simplify the task, not omnis, nor planars.