I'd try doubling it. *There isn't really any way to have too large a value
(until you get so large that it takes some time to charge up when you put
the key down, and that's many orders of magnitude larger). *However, big caps
are not cheap and I suspect you'll see diminishing returns pretty quickly..
--scott

Actually that's not true with plate modulation. The coupling cap
bypasses B+ audio through the pi network L, the safety RFC and/or a
DC path from the antenna to ground.

John, KU6X