I've all tubes. I do find that they have a more pronounced three dimensional soundstage. It could be that they simply have a lower noise floor—allowing the tiny signals necessary for the most nuanced sound cues through. It could also be that the glass picks up sound waves which causes a slight reverberation giving the music a more echoic perception.
Monoblocs sound better because there is no cross-modulation between channels. In other words, you get better stereo separation.