OK so I found the problem. V1 is a crappy tube. One side of it was failing when it heated up. I slapped in a JJ, used DeOxit all up in there and the other pre tubes. I played it for an hour and a half and no issues. Just awesome. V2 is a hissy POS. The stock pre tubes are Sovteks which I have always, always hated.
And yes, it sounds really, really, really great.
Sounds like you've solved it, but in case anyone else comes along with a similar problem, thought I'd still post this.
I'd had problems with volume drops on my SB300 lately (no crackling though), through a couple of different cabs. It's still under warranty, but thought I'd start by checking the easiest thing first - the speaker cable Fender supply with the amp. Replaced it with a new one, and whatdya know, it's working fine! Other people earlier in this thread had also mentioned having issues with their cables, so they're obviously not great quality.
