Another noteworthy piece of information I've come across is that Victoria and the Weber 5F1 kit use a 25uF capacitor in parallel with the 1K5 resistor coming from pin 3 of the 12AX7.
Here's the Weber 5F1 layout:
http://taweber.powweb.com/store/5f1_layout.jpg
And a short film from Victoria humorously illustrating a 5F1 build:
http://www.youtube.com/watch?v=1xj6d9MLCpo
Supposedly this is a bypass cap, added in order not to screw up the 12AX7 bias as far as DC goes, but boosts the mid and low frequencies (or rather cuts some of the high ones?) in the AC current.
In any case, I don't have a quality electrolytic capacitor to add there, and I'm fed up as it is with waiting for parts so I'll just try the 5F1 I have as soon as I can, and if there'll be any problem with the tone I'll make that adjustment then.