OK, this was something that occurred to me recently on this board. Here's my through process:
1. Modern recordings (particularly CD's), strip out harmonic information that makes recordings sound sterile.
2. Tubes restore some of the harmonics in the playback chain.
3. Conclusion - Tubes improve upon the original recording.
This means that tubes are not "neutral", or "true" to the recording. But it also means that tubes are more true to the original performance.