I tried a triode-MOSFET cascode as the voltage amp section in my prototype Merlin amp. It did quite well. There is a warning which I did not heed when I first assembled and tested the circuit.
Thou shalt protect the small gate-source voltage limit with a Zener clamp!!!
I installed 18V/500 mW Zener clamps on the good channel, and installed the same when I rebuilt the other channel this morning.
The cascode turns the amplifier triode into something with horizontal plate lines at the Ec2 voltage. Take the triode plate curves, draw a vertical line where the upper gate voltage is set, and at the grid line intersection with this vertical, draw horizontal lines out to the maximum Drain-Source voltage. It is a whole lot easier than accounting for the curve of a triode's plate characteristics when drawing the composite cascode curves.
It also leaves a maximum fraction of high Z operating window. It approaches B+ - Ec2, instead of B+ - 2*Ec2 for equal triode sections on a more traditional cascode.