So, I just sold my old guitar amp to a mate (40watt JCM2000 combo) after getting it re-tubed and he's run into a bit of a snare with it. To begin with it worked and sounded great, but after a week or so it's suddenly gone super quiet. With the master, channel volume and gain all at full it's making sound but only barely. Nothing at all out of the clean channel. Any idea as to what this could be? Fuses are fine, tubes are all seated properly (though one of them didn't look new to me, but none looked noticeably blown) and we've tried a few leads and different guitars.