Yes, you are on to it! When only the 470 kOhm resistor was there from "signal hot" to ground, then the entire signal voltage being fed into the amp was developed across it and thus fed the control grid of the pentode. When you added the smaller resistor from the previously grounded end of the 470 k, you created the divider and the Voltage across the smaller resistor became the one fed to the grid.
Here's an easy experiment which may give you a ballpark to work from: If you obtain another 470k resistor and clip it across the existing one (parallel to it), you will essentially cut its value exactly in half to ~235k. This will change your Voltage divider ratio from roughly 10:1 to roughly 5:1. In other words, it will reduce the attenuation to about half of what it is now. If that moves things closer to where you want to be, then you can measure the resultant paralleled resistors with your Ohmeter (approx. 235k), and clip a similar value across the combination. The typically available values of 220k, 240k, or even 270k will be close enough for this third parallel. Anyway, you will have reduced the Voltage divider ratio (and, the attenuation) by approximately half again.
Once you find the combination you like, then you can measure everything with your Ohmeter again and easily calculate the Voltage divider ratio you like. Once you've done that, you can decide what you want to do about making the divider permanent.
Two things to keep in mind are: First, the total resistance of this Voltage divider to ground is essentially the input impedance of your ST-70. While the original spec calls for 470k, you can get away with quite a bit of deviation around that value, providing that you don't lower it so much that you overload the output of your preamp. IIRC, the output of the Foreplay is around 600 Ohms, and the general rule of thumb is that you want to drive at a minimum 10 times that, so 6000 Ohms is your lower limit for input impedance, and I seem to recall that Mr. Joppa wouldn't mind something higher. The upper limit is pretty arbitrary, but you can go higher than 470k for sure.
The second thing is that this input impedance is where the signal is developed for your amp, so it never hurts to make this a nice-quality resistor or combination of resistors. Metal-films are my favorite for this duty, but carbon-film could work very well, too.
As far as your other issue is concerned, I wonder if this gain imbalance is a condition you can create on demand? Can you make it happen and keep it that way long enough to take measurements? That will make it a lot easier to hunt down, I'm thinking.
