I asked a friend who dabs in synths, what the vacuum tube was doing in Tsumugi's station.

The answer was:

Very likely a korg product. They added a tube to the electribe series, and the triton extreme.

tbh, it's kind of a gimmick. They noticed guitarists pay silly amounts of money for vintage tube amps, and cashed in on that hoping synth players would think the same way.

Indeed, I went back and re-capped the performance at the opening celebrations, and Mugi was playing a Korg Triton Extreme. What's interesting, it is not at the Korg's website anymore, being released only in 2005. Perhaps it was too expensive.

A site about old synths says:

The most extreme model [of Triton family], it quintupled waveform memory up to 160 MB with 120 voices of polyphony. Most of the Triton series expansion boards have been pre-installed as standard in the Extreme, while many sounds from the older Tritons have been improved as well. Sample memory is also upgraded (up to 96 MB of sample RAM). And in addition to its new "paint-job" a major feature was the addition of a genuine 12AU7 "Russian Bullet" vacuum tube which could be used as either an insert or master effect, or simply by itself to allow for warmer, guitar amp-like sounds and for more extreme analog overdriven/distorted sounds.

The picture at the site is of a shorter keyboard. I think Mugi actually got the longer version, like the one pictured at Wikipedia.