my concern is that during that millisecond of switching, that little glitch or pop could take out $100+ worth of tubes (or worse, your speakers). For that I can wait a minute or two to shut the amp down, let it cool off slightly (don't want to pop the HV rectifier when you turn it back on hot) make the switch, then power up.
It's been said a thousand times, you'll have more bass and treble (and more power) in ultra-linear,
but you'll have a better midrange in triode. The difference is very similar to the difference between
the sound of KT88s and EL34s.