So I can’t fairly compare the 6xx with the LCD-2C as I haven’t heard the latter, plus they are 2 very different headphones. I can say that yes, the 6xx gain significant bass extension and impact when driven by a tube amp. They don’t turn into subwoofers on your head per se, but they are a very different experience on tubes than with solid state amps.
I’ll shamelessly plug my own 6xx review. I talk more about their transformation on tubes within: