Thank you all for replies. I have finally had some time to sit down with the amp and schem and try to sort it out.
For starters, I took voltages again with no tubes in place, in simulclass/ pentode setting (on the clean channel, if that affects anything). The EL34 sockets read 495, 493, and -38 on pins 3, 4, and 5. The 6L6 sockets read 495, 0, -44 on pins 3, 4, and 5. If I go just upstream of the resistor on pin 4 of the 6L6 sockets, I get a voltage of 494.
After reading lovetoboogie's response that my bias supply (pin 5) should be between -50 and -60, I checked voltages from the diode to the distribution strip for the sockets. I found a schematic for the power supply in post 4 at this link http://music-electronics-forum.com/t41204/
I began checking after diode D15 (-69.2V), R105 (-67.1V), and R233 (-59V). The last V should read -57V, according to the schem, but I assume those voltages were measured with the tubes in. From here, I jumped over to point E in the output section schematic which precedes a 220K resistor to each side of the output section. These 220K resistors are dropping the bias from -59V to -44.9V. R105 and R233 have the same amount of current going through them (about 2.5 mA), but using ohms law on those 220k resistors give .06mA per side, so .12mA total. Is this disparity showing where my current is going?
After checking these voltages and thinking, I had another thought. Since I have been careful to not mix up tubes except to test if it was the one EL34 or the socket, and every voltage was the same between each socket/ side, I decided to test the 6L6 partnered with the bad socket. After changing it to the other 6L6 socket, the opposite EL34 socket began to display the red plating issue, while the previous offender played nice.
I switched 6L6s to a known good pair, and the problem took longer to occur, but happened after turning up the volume some and playing with the SC/ClassA switch. Should I be concerned about the bias voltages I am getting? With new tubes all around, I am getting 485V, 481V, and -38.3V on the EL34 sockets, 485V, 0V, and -44.9V on the 6L6 sockets, and 486-487V on the other side of the resistor on pin 4 of the 6L6. That 0 voltage reading on pin 4 is puzzling me, as the schematic makes it seem like the voltages on pin 4 of every power tube socket should be in the same ballpark. Perhaps that is the rock I need to uncover some more to solve this issue?