Some further impressions (based on my ears and preferences):
- Classic 99 is much better than the M50X. The M50X sounds comparatively scooped, and buzzy on the high end.
- Classic 99 is better than the R70X (which is open back, and about the same price). R70X was the best headphone I had heard under $750 until I heard the Classic 99. The Classic 99 has a bit less control of bass and slightly smaller stage, but more accurate mids and smoother highs. Detail of both is similar.
- Not surprisingly, the LCD-3 has significantly better sound quality than the Classic 99. While the signatures have similar warmth in terms of lack of timidity of bass and roll-off of highs, the LCD-3 has much better shape and control of bass, smoother overall, more detail, more realism, and bigger stage. But the comparison isn't fair at all, since the LCD-3 is more than 6X the price, open back, heavy, and requires much more power to drive (Classic 99 is among the most efficient headphones I've run across, but does sound better with a better DAC/amp - I've tried the iPhone, DF Red, Mojo, and Hugo 2).
If I needed a closed-back and my budget was say $500 or even maybe $750, I'd gladly buy the Classic 99 and consider it an excellent value. I'm actually looking for good closed-back, and my budget is more like $2K, but I may still buy the Classic 99 anyway.