pinconnection wecp 20
The power stage circuit is clear so far. With help of the 1 ohm resistors, you should be able to adjust the bias currents. The guys who invented the KT66/88 combination may have an idea about an appropriate bias point. There are many possible settings, I think. Also the two types have different plate dissipation rating respectively maximum current.
If shorting the FET doesn't remove the switch click, a different interference coupling path may exist. (I assume, that you applied the short to the same ground that's used by the FET!). Then the FET on-resistance probably doesn't matter.
BS250 can't be used, cause it's an enhancement transistor while JFETs are generally depletion transistors, "on" without a gate voltage, "off" with a positive voltage. Also MOSFET are unsuitable, cause they have a substrate diode shorting positive Uds. Original J175 should be preferred (if the the On-resistance is an issue, which hasn't shown clearly yet, to my opinion).
Regards,
Frank