Disappointing to hear it's a solid state pre, but it sounds great as it is, it's like a hybrid amp done the right way round! Even though the LB30/CTM30 is all tube, the LB I had a couple of years ago sounded very bright and brittle compared to this, not helped the fact it had no gain control. The 15 is perfect for bedroom/studio use, I would consider getting a CTM100 for rehearsals/gigging in the future.
Anyway, I took it into PMT today and couldn't reproduce the crackling issue at all - it had been getting better since day one and has now almost disappeared completely. The noise levels were the same as their shop model, ie pretty quiet but not silent with gain at min and master at max, and getting very noisy with the gain cranked. Bit of a wasted trip in the end, but good to know that mine isn't any noisier than it should be.
Might try some replacement EL34s down the line but happy with the stock ones for now.