I don't like this vintage jFETs, because this FETs out of production since a long time.
There are various good available p-channel jFETs like this:
http://www.onsemi.com/pub_link/Collateral/MMBFJ175LT1-D.PDF
http://www.onsemi.com/pub_link/Collateral/MMBFJ309LT1-D.PDF
By creating a darlington or sziklai darlington with an additional BjT small signal transistor like 2N4401 I don't see basically disadvantages. OTOH - nobody uses this possibility.
Why?
The thread
http://www.diyaudio.com/forums/parts/105338-2x-2sk170-place-2sk389-2x-2sj74-place-2sj109.html
don't provide informations.
There are various good available p-channel jFETs like this:
http://www.onsemi.com/pub_link/Collateral/MMBFJ175LT1-D.PDF
http://www.onsemi.com/pub_link/Collateral/MMBFJ309LT1-D.PDF
By creating a darlington or sziklai darlington with an additional BjT small signal transistor like 2N4401 I don't see basically disadvantages. OTOH - nobody uses this possibility.
Why?
The thread
http://www.diyaudio.com/forums/parts/105338-2x-2sk170-place-2sk389-2x-2sj74-place-2sj109.html
don't provide informations.