If you have a Steelhead-

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.

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.

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.

Just out of curiosity, in the Herron what tube type does the 7DJ8 replace?

The 7044s, or any of the several congeners of the 7044, in the Steelhead are not "rectifiers", first because 7044s are dual triodes, not rectifiers, and then because they serve as White Cathode Followers in the Steelhead, which is just a fancy cathode follower. 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.) The low output impedance of the cathode follower then makes it possible to drive either the volume control in the Steelhead, if it is being used as a full function preamplifier, or the outboard linestage, if the Steelhead is used as a phono only stage. In the case where you are using it as a full function preamplifier, there is a cathode follower that drives the volume control and then a second identical cathode follower that is driven by the volume control and outputs to the amplifier. So, the output stage sequence is: CF - (coupling capacitor) - volume control - CF - (coupling capacitor) - amplifier. That’s why, if memory serves, there are two 7044/6922s per channel, one for each White CF.

The 7DJ8 is by all accounts a nice tube, but bear in mind that it is designed for a 7V filament supply, not the 6.3V supply that is standard for filaments of most dual triodes and which is present in the Steelhead. Fortunately, it works on 6.3V, apparently.

For the single amplification tube per channel in my Steelhead, I substituted some Siemens CCa, happily, but I won’t say it’s a night and day difference from OEM tubes. I made some changes to the output section I described above, in terms of the output coupling capacitors, that made a much bigger improvement in SQ. Note that the signal must flow through two coupling capacitors on its way out to the amplifier, if you use the Steelhead as a preamplifier.