The stock pot was a bit noisy over part of its range, so I replaced it with a same-value Alps Blue (from a previous T-Amp build).

Unexpectedly, this also provided a sonic upgrade - midrange and highs noticably tighter and more coherent. Not quite as dramatic as the CCE mod, but certainly well worth while!

I suspect you could go on like this, component by component, gradually approaching the sound of the better specced Zens. That's me done CCE, V-Caps and the Pot and boy it is sounding very very clear, coherent and musical.

Following changes to my source (Valab NOS DAC) I tried going back to my Charlize T-Amp for a day. Although I like the totally silent background, instant-on, and zero heat, the sound is competent, but clinical and uninvolving. No contest.

Fair points.

Re the tube stage - I'm sort of reluctant to keep adding to the signal path.

btw I still have a hum problem, which I suspect is coming in the mains earth (or power). Hard to find.

Amp by itself, powered up, is quiet (minimal constant background hum at all volume levels.) RCA connectors + TVC gives added mains hum, none at zero volume, becoming louder as volume is turned up. This is peculiar because the TVC is not powered, and has no inputs attached.

There is no hum using other amps (push/pull, T-Amps) - I understand that SETs are very sensitive to power supply/earth contamination.

Amp by itself RCA to DAC (no inputs) - exactly the same mains hum.

Amp by itself - direct to CD player, nothing else, exactly the same mains hum.

Changing from silver to shielded RCAs (Van Den Hul D102III) makes absolutely no difference.

Appparently simpily adding an earth spike is a bad thing to do - you should get an electrician to check the Ze of your incomer earth and your bonding and sort things out properly for the whole property.

Sigh.