@lewm Cathode followers convert voltage to current while dramatically reducing output impedance and adding no gain to the signal. (Which is why it makes little sense to sub the stock tubes with very expensive equivalents.)
My experience is that changing tubes in a cathode follower position does have an impact on sound. I tried different tubes in the cathode follower on my old Jadis preamp - the change in sound was significant.
Similarly with my Marantz 7 - for example long plate mullards sound quite different to flat plate Telefunkens in the cathode follower position.
I would also assume low noise, matched sections would be an advantage, particularly if the cathode follwer tube is shared between channels.
What caps did you upgrde to in the Manley ?