If you have a Steelhead-

I forgot to mention that I think the Steelhead is a great unit, made greater by a few mods.  But if you are using it as a phono only stage, then my upgrades would do less for the SQ.  The very same CF and coupling capacitors that couple the output of the phono section to the volume control, when the Steelhead is used as a preamplifier, are instead diverted to connect to the output RCA jacks, when you use it phono only.  Therefore, you could consider upgrading only those coupling capacitors, if you care to do so.

@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 ?

You know what, Dover? I wouldn’t argue with what you say about CF tubes. When I wrote what I wrote, it might have been in spite of my own experience. At least where I got rid of a 12AU7 and replaced it with any electrically similar substitute, e.g., 6CG7, 12FQ7, ECC99.. I’ve done that more than once and always thought it beneficial. The fact that the CF provides no gain would lead one to believe otherwise. But I don’t know about for example replacing a 6922 with a more expensive 6922. At least I wouldn’t bother.

Lest I be chastised as I was once before by Dover, for suggesting incompatible tube substitutions, I hasten to point out that although the 6CG7 and 12FQ7 (and the 6FQ7) are identical to a 12AU7 in electrical parameters, one cannot blithely sub them for a 12AU7 without first doing some very minor re-wiring of the tube socket.  You would also need to know that your unit supplies 6V to the filaments, in which case the 6FQ7 would be the choice, not the 12FQ7. On the other hand, the ECC99 has the very same pin-out as a 12AU7 and can be plugged into a socket wired for a 12AU7, but the ECC99 has a higher transconductance and lower plate resistance than a 12AU7, which only makes it an even better CF.