Rectifier socket does not matter, they are wired in parallel.

As for the rest, it could be several problems, considering it's a newly assembled kit.  We start with reading voltages.  What is the B+ coming off the copper cap? And what is the voltage between pins 7 and 3 on the EL84 tubes ("Actual Plate Voltage")? And what is the voltage across the cathode resistor?

Everything else stock to the layout?

It should not.  The copper caps are designed to give a similar voltage drop and current to their vacuum tube equivalents.  A WZ34 should perform the same as a GZ34, except they don't have any filaments, so they don't require a 5V supply (but can be used in place of tubes that do).

Vox and Matchless are famous for running tubes hot and killing them young, but tubes don't red-plate until they are near 200% dissipation...which is too high.

I bet it's an issue with the switching of the cathode-bias resistor(s) and the High/Low switch.

It's designed so that when you are in High (4xEL84), you should be running only the 62-ohm, 10-watt resistor. 

When in Low power mode, the switch "turns off" two EL84s and it adds the extra 68-ohm, 10-watt in series for a total resistance of 130 ohms to rebias. 

Sounds to me like the latter isn't happening...it's turning off the two tubes, but not changing the cathode-bias resistance.  It probably is a problem with the switch wiring since you said it was working upside-down.  You may even have it working in reverse.


[edited from before]
You shouldn't even need to power up to check it.  Looking at the Ceriatone Layout, check the resistance between 1.) where "O" connects to the 62-ohm resistor  and 2.) Ground.  You should have 130 ohms in Low switch position and 62 ohms in the High position.
[edited from before]

It's possible, it means you are getting no screen voltage on that tube.  But the affect should be triode mode, which wouldn't necessarily cause red plating and shouldn't affect the other tubes.

Check those resistors; then measure everything I noted above and calculate dissipation...
http://www.webervst.com/tubes1/calcbias.htm

Well, it has to do TWO things...turn off two tubes and rebias. 

Here's a pic of an UPSIDE-DOWN chassis...

<<<<<  CHASSIS BOTTOM >>>>>>

<<<<<  CHASSIS TOP >>>>>>

So, the "O" is the top of the chassis, closest to the tubes (directly behind the word LO).

Note on the layout, where it says Chassis Bottom below the switches on the layout.  If you have reverse the wiring, you should probably check your speaker switch as well.

Here is a schematic...
http://www.prowessamplifiers.com/schematics/post/matchless_dc30.pdf

Note what happens at switch behind the power tubes.  In "Hi", it connects both X and Y to the Cathode resistor/capicator, turning on all the tubes...and it sends them out through the 68-ohm resistor to ground.  In "LO" the circuit Y cathodes are disconnected and the outer tubes are taken out of use, plus it forces the X tubes cathode through the 68-ohm resistor before it reaches the 62-ohm resistor and then goes to ground.