Good to see you again, Tony!
Silver shouldn't matter but the tube outputs are pretty high-impedance and thus more load-sensitive so a cable with lower capacitance can help. I used special coax in the past to get the capacitance down in the 10 pF/foot (~30 pF/m) range, down from typical cables' 20 to 30 pF/ft (60 ~ 90 pF/m). And short cables... Ditto the transformer, changes in core material and winding probably swamps wire contribution, but a custom design could certainly be better than stock. A lot of stock transformers include small capacitors for RFI suppression and those can affect frequency response.
JFET's have a tube-like transfer function so should sound very similar. May have lower noise but probably overload sooner (not normally an issue for a microphone, I have seen that mostly with turntables).