You should research how the many "channel switching" amps do it. Most are not channel switching at all but rather an insertion of a gain stage (or two or even three) just as you propose. The basic idea is the same with most. Certain senarios do require special attention and I don't know your schem. Your basic idea is correct , that is, you would shunt the unused gain stage grid to ground. Sometimes it's also possible to shunt the unused stage output to ground. This may work out better for some designs strictly as a matter of convinience with the switching system. Again, I don't know your schem so I can't comment on specifics. I will add that you need to pay attention to a couple of things. One is that when switching you need to have a 0V reference on both switch ends to avoid noise due to a voltage differential. Another is that you must make sure that if you are using a switch on a grid that it never "floats" the grid eliminating the tubes bias condition. Like I said, look up some schems for channel switching amps and you'll see these things in practice. You may find a design that will swap easily into your plan. The switching devices used today are typically LDR's and they are very effective for this sort of thing but if you plan to use relays there are plenty of schems to represent that too.