805 or 845 is a king in the tube world,and this time, the amplifier is the set one,which make the sound even beautiful.i am doubt before that, op do have a chance to listen 805 or 845 amplifiers? If that does not happened before, it is the reason you found it is surprising good this time.
But there are 2 drawbacks here,
1, set amplifier has less power,which is not good for some low sensitivity speaker.
2, the signal to noise rate is way low compared some mainstream amplifier.
For example, luxman could search 117
While 805,845 set amplifier is way below 100.