Two Channels, One Volume, One TMB Tone Stack - Mixing Resistors.

[Chuck H]

Not sure what you mean with shunt resistor.
As the VE grid has low AC impedance to ground (that why it's called a virtual earth node), a large value resistor from grid to ground should not significantly change the gain.
Gain is given by feedback resistor divided by input series (mixing) resistor + source resistance.

Sorry Helmholtz. By shunt I meant the feedback resistor. This method of NFB is sometimes called "shunt feedback" so my terminology above is generally understood by most. The only resistance to ground at the VE grid would be the reverb recovery output circuit and it's volume control. Which remains constant at 500k.