狼兄，我在网上帮你找了这么段解释，不只能否足以释疑。
不好意思，懒得翻译，不过要是你。。。
There are several main reasons that makes tubes sounds better , the following list out those points,
1. Tube is a much more linear device than transistor during operation.
2. Due to the high non-linearity , conventional transistor amplifier must have very heavy NFB (negative feedback ) to make it work stable, but tube amplifier can work well with very little or even no NFB, and heavy negative feedback is well known today as one of the main reason of poor sound quality.
3. Tube is a totally mechanical device, inside a tube is just some metal plate arrangements, the effective operation area is much much larger than a semiconductor (sometimes even 1,000,000 times larger). Almost every gold-ear audio engineer should knows , when music signal pass through a very small area, it will implies certain squeeze feeling to the signal itself and it is very noticeable , so when a signal just pass through one transistor, you will immediate sense the compressed , flattened and hardened feeling from the music. The more transistors , kills the easiness more and give your more flatness and hardness to the music.
4. Tube-amp has higher THD (total harmonic distortion) than transistor-amp by testing, but those THD created by tube-amp is all even harmonics which is very pleasant to ear and will not feel disturbing unless it is > 12% . But transistor-amp gives out odd harmonic distortions which is highly un-pleasant to human ear, by actual experiment, just 0.2% of odd harmonic distortion is audible and will create un-pleasant feelings even to un-trained ear.