I understand what bothers you about FA-003/clones. The same things bothered me as well and that's why I eventually sold them. They are not dark, but they lack detail and definition. I felt that there were some very noticeable reflections from the housing which contributed to that muffled feeling you write about. I heard it too and while it wasn't a deal breaker for me, I found better sounding closed headphones in the same price range, such as Shure SRH840 and AKG K271 which sound better damped and more focused than FA-003 to my ears.
I like Beyer sound very much too, although I think AKG sound actually fits my tastes best. Beyer's tend to be smoother sounding than Sennheisers and AKGs and remind me more of Orthodynamics in this regard. Their closed DT770s are somewhat grainy and harsh in the treble though, but DT880, 990 and T1 are very fluid sounding headphones. Beyers also tend to be highly detailed with more apparent resolution than Senns and closer to AKGs in this aspect. Beyers are also more laid back/distant and less clear than AKGs, especially in the mids, where even Sennheiser cans sound more aggressive. But the laid back nature doesn't prevent Beyers from retaining a reasonably lively character, probably due to the high amount of detail, accurate tonality and great articulation, but also their often forward and emphasized bass and/or treble. One of the main drawbacks of Beyer sound for many people tends to be their bass and/or treble boost and comparatively recessed mids. Some also complain about a lack of texture and richness in the sound, especially in the mids and a thin, cold character (similar issues as with AKGs). Personally, I often agree that midrange reproduction is not one of Beyers' strengths and that the treble on DT**0 series tends to be somewhat too hot (but not harsh). I disagree with those who think that Beyers are overly colored though, as their tonality sounds very natural to me overall and more neutral (and perhaps more natural) than that of Sennheiser cans overall IMO - the latter being on the warm and thick side of neutral. I think AKGs do tend to be a bit more true to life, especially on acoustic recordings, than headphones from the other two companies with that special sense of clarity in the mids and a fast attack that give an impression of extra liveliness. For electronic, trance, techno and stuff like that, I think Beyer signature works more effectively, while Sennheisers work best as all arounders.