No. It's magic.
JK. It's electrical engineering.
I'll give this a shot. Maybe @Low F# will agree, maybe not. The saturation of the transformer, especially when hit by a hot signal, introduces more midrange and upper frequency harmonics. The coloration is quite pleasing, and because the harmonics are more present in the mids and upper echelon of the highs, to me it almost sounds like a slight scoop in the lower trebles or even, depending on how the high frequencies are colored, a high-end roll-off .
Transformers saturate at low frequencies far before they do at mid or high frequencies. What you're hearing is most likely due to interactions (resonance) between the inductance of the transformer and the interwinding capacitance and the cabling capacitances.
Anyway, if you like what it does, it doesn't matter how it got there - if it makes you like the sound a bit more, then it's done it's job.




