This was just a schematic I tossed up there. I used the DN2540N5 when I active load a triode. This is a depletion mode lateral MOSFET. I normally don't cascode them. I use one per triode, it holds the tube, but not as stiff as the BJT's would, hence the lifeless sound. I let the tube swing a little. Using a tube as CCS is acceptible I take it, well it works for me, with this combination I am able to broaden the midrange focus tubes are famous for further into the bottom and top region of the audio spectrum. No more flabby bass and loose treble.
Analag