At a more practical limit, we have Guinevere.

Merlin is fairly high input Z, basically as set by the grid leak of the driven triode. I think I drew this at 100k, but 47k resistor would be a good suggestion, and less might be even better. The cascode shields the grid in a similar way the screen grid in a pentode does, so we don't have a triode's Cg-a multiplied by Miller capacitance.

I take Wayne's mention of noise resistance as important. If we're going to drive Merlin with a pre like Guinevere, we can probably cut the input grid leak resistor to 20k or less w/o troubles. I am going to get out the Iron and try some of this on my amp as soon as I can.
regards,
Douglas