I just got my LSC last month, having been a devout Marshall guy for a long time (but getting tired of dragging around the half-stack). I was excited about the easy tube swap feature, and I tried it almost immediately with a set of Mesa-branded EL34s (Sovtecs, I reckon).
To my surprise, the EL34s were a disappointment. Compared to the 6L6s, they were flat and colorless. I switched back after one night of wanking on them, and I'll keep them around for another good burn-session; but I was surprised at how little they gave, compared to the responsiveness of my old Marshall.
The 6L6s, on the other hand, have been a dream and actually I have been able to dial up a pretty good Marshall clone tone with them. Not quite the same breakup, but with a really similar "crust" on top, and super responsive.
So, personally, I'm happy with the 6L6s, even though I bought the thing with the swap in mind.