I would consider the LCD-X almost the polar opposite or the HD-800s. 300b tube amps are reported to match perfectly with the HD800s since they provide the warmth that they lack; now, the LCD-Xs are already darker-sounding and much warmer than the HD-800s. Wouldn't it be reasonable to assume, then, that the 300B + LCD-X combo would be too syrupy or overly warm? I've never actually listened to a 300B amp, I'm just going from what I've read in the head-fi threads.Yes
I haven't had too much experience with other 300B tube amps other than my ECBA and some I've heard at meets. The LCD-X with ECBA is not remotely overly warm or too syrupy to me. I have heard other tube amps that sound that way though, not necessarily 300B. I agree with PrTv that there are several factors that contribute to the sound, not just the fact it's using 300B tubes. The general character of the ECBA is transparent, detailed, spacious, fast - among other things. That doesn't change with the LCD-X, and it doesn't make the combo sound overly warm even though the headphones are considered darker as you say. In my experience using a good tube amp (300B or not) does almost always add some level of fullness, liquidity, and naturalism. And it can still sound fast and highly resolving.
I'm not saying other 300B amps don't sound too syrupy but this shouldn't be a broad generalization with the LCD-X.