Trainwrecks are designed to have a very sensitive harmonic response to input volume differences. As the volume increases one side of the push-pull output will have a longer period than the other. Try turning the guitar volume down -- quite a bit. And then, turn the amp volume up -- quite a bit. Then go back and experiment with your guitar volume. Trainwrecks are unique in that they were designed to exaggerate this increasing long-short signal wave period as the input increases. As a result, the harmonics/timbre will change quite radically as the input volume increases. If the amp is already pushed to it's limit -- for input -- the distortion can be unpleasant and unexpressive. That was my first experience with a Trainwreck. I couldn't see what was so special about it. I just thought it was just a bunch of unpleasant, unexpressive distortion. I believe the Trainwreck was designed to go after the edge-of-breakup, don't-need-no-stinking-pedal, kind-of-thing. But, in a very dynamic and extremely expressive way. A Trainwreck devotee might explain it better. Extremely touch-sensitive, borderline stable, borderline chaotic, not so pedal-friendly. It sounds best when moving back-and-forth over that edge-of-breakup, IMHO.
now I have another to check out