My phono pre-amp uses two 6SL7 in the gain stage and an 12AU7 for the
buffer output stage.
Can someone explain the role of the gain vs. the output and which affects
the sound more?
The gain stage takes the very small signal from the phono cartridge and amplifies it to about 2V, which is the same level as a tuner or CD player puts out (it's called line level). A regular preamp expects to see voltages at this level. The phono EQ is also usually taken care of by the gain stage(s).
The output buffer isolates the gain stages from the interconnect cables and the input of the regular preamp. It allows the gain stages to operate in an environment that best suits them without any influence from outside.
It's not really possible to tell which has more effect on the sound in general. It's going to vary from design to design.
