Did you ever get to the bottom of this? I only ask as I have UK-made LB30 #27, and mine stays clean (and gets very loud) using the "high" input, regardless of whether I play an active or passive bass through it. The "low" input gets dirty almost instantly, so much so that I don't use that input at all.
I've checked the valves in mine (JJ ECC83S x 2, JJ ECC82 x 1 and JJ EL84 x 4), and I'm going to experiment with some ECC81s instead of the 83s in the preamp stage, as per Voxpop's suggestion, soon.
Cheers
Gareth