The "softer sound"of tube amps is due to several things:
1) Limited transient velocity. A transistor amp can go from -max to +max much faster than a tube amp (with less overshoot as well).
2) Lower bandwidth. Most tube amps are limited to 20-20kHz or 20-40kHz. Many transistor amp have bandwidth of 0-200kHz or more This is a direct result of #1 above.
3) when a tube distorts, the harmonics are all "even order" which sounds more "musical than the "odd order" harmonics of transistors

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson