This article explains it pretty well.
http://www.moxtone.com/Tube_buffer_en.html
BAT uses tubes in the rectification (power supply) vs a SS rectifier. This is technically a quieter approach. My experience is that in practice, this is only really critical with highly sensitive speakers.
They then use tubes in the gain/amplifier stage and the buffer. Alternatively, Conrad Johnson uses two 6922s in the amplifier stage and then MOSFETs in the buffer so yes, this is a hybrid approach.
Many companies take very different approaches and each designer often has very firm opinions on what does and doesn’t make a good tube circuit.
I have had one designer tell me a 6922 is not appropriate for audio applications. I have other designers where it is their preferred tube.
The important thing is picking a preamp that will marry well with your power amp.
http://www.moxtone.com/Tube_buffer_en.html
BAT uses tubes in the rectification (power supply) vs a SS rectifier. This is technically a quieter approach. My experience is that in practice, this is only really critical with highly sensitive speakers.
They then use tubes in the gain/amplifier stage and the buffer. Alternatively, Conrad Johnson uses two 6922s in the amplifier stage and then MOSFETs in the buffer so yes, this is a hybrid approach.
Many companies take very different approaches and each designer often has very firm opinions on what does and doesn’t make a good tube circuit.
I have had one designer tell me a 6922 is not appropriate for audio applications. I have other designers where it is their preferred tube.
The important thing is picking a preamp that will marry well with your power amp.