I am getting an odd sound when I use moderate to high gain amp models with my spider valve mk2 112. Chords seem ok, but there is a strange "yodelling" on single notes which most noticeable around 10th fret upwards. Slow solos don't sound good. I mainly use the plexi model for heavier songs and all lead work, but the odd sound is present on all high gain models. It occurs with the Divided by 13 model, but is nowhere near as pronounced, so I am thinking that it's a preamp valve issue. This happens with all my guitars, Fender Strat with Seymour Duncan STK 4's, regular Fender Strat, Gibson SG and Strat clone with Jeff Beck full size humbucker.