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Just purely for the 6V6s, the PI's tail and cathode resistors need to be changed from 56k/820 too 20k/470. This will help get more drive to the 6V6s, which have a lot less gain than EL84s. If your amp is a more recent Trinity Plexi, it may already have the 470 ohm in there, rather than the more usual Marshall 18W's 820 ohms.
The question then is how to reduce the drivel level for the EL84s, so they don't start crapping out tone-wise. So here's what you end up doing, if you want to run both 6V6s and EL84s optimally. Instead of both sets of tubes using 470k grid reference resistors, you split each 470k into a 220k and a 250k, connected in series. So you end up with a 220k+250k (=470k) on each side. You connect each EL84's grid to the junction of it's respective 220k/250k pair, so that it's fed an attenuated version of the PI drive signal. The 6V6s are connected as before, to the top of the 220k+250k pair, via an 8.2k grid blocking resistor, just as if it was a regular 18W amp. I hope that makes some sense. It's actually simpler to do than to describe.
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Great tones come in small glass jars!
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