Hi Bart,
Malcolm's comments are spot on. You can take the output from the SS (first) section of the GK1, and this has very low impedance, around 30R, which gives it a LOT of drive and a small amount of gain (just over 6dB). If you want a volume controlled output, however, you must take it from the wiper of the control, which of course has much higher impedance, set by the lower resistive arm of the 50K pot itself.
You could add an IC buffered output which was volume controlled using something like an HA5002 from Harris, or even a discrete, SE emitter follower; but this adds complexity and may be overkill. Up to you!
Mal, thanks for your 100% accurate response!
Cheers,
Hugh