I haven't heard the Omega speakers, but I am pretty familiar with single driver sound. I'd say in general the highs will not be rolled-off, unless your amp is doing the rolling off. But the bass will be somewhat limited in how deep it goes, and there won't be the same amount of "slam" that many people are used to. Also, there could be some forward character to the upper midrange, with certain single drivers. And there will be a rather narrow "sweet spot" for listening.
However, many are quite willing to accept some of these characteristics, so they can get the good points of these kinds of speakers. It is an individual preference, and I would definitely recommend auditioning this kind of speaker before buying, because it will be quite different than most people are accustomed to. I prefer it, but I am not everybody, and have rather different ideas about my sound, than many. I personally believe that this kind of speaker requires an analog source, because of its very revealing nature that will not cover up the digital problems. Also the efficiency levels will show up any noise or hums you might have in any part of your system.